Our English word pornography is from porneia (illicit sexual activity) + graph (a writing). Thus pornography ("porn") is a writing (picture) related to illicit sexual activity of some variety.

Expositors Greek Testament writes that...

The term porneia (fornication) is to be taken in its proper sense and is not to be restricted to any one particular form—the license practiced at heathen festivals, concubinage, marriage within prohibited degrees, or the like. The moral life of the Greco-Roman world had sunk so low that, while protests against the prevailing corruption were never entirely wanting, fornication had long come to be regarded as a matter of moral indifference, and was indulged in without shame or scruple, not only by the mass, but by philosophers and men of distinction who in other respects led exemplary lives.” (Nicoll, W Robertson, Editor: Expositors Greek Testament: 5 Volumes. Out of print. Search Google) (Comment: Does this comment remind you of any modern day society?)

Christianity brought chastity, a virtue that was foreign to the Greco-Roman world. Into this world of pagan idolatry where sexual immorality was not only condoned, but regarded as normal, the Christian faith came as a purifying fire. Twentieth-century America has reverted back to the “normality of sexual immorality” and the revival fire of the Christian faith is desperately needed.

Remember the context of the society Paul was addressing. In the Greek culture of that day, prostitution and fornication were considered permissible activities. A married man in Greece cold engage in extramarital sexual intercourse as much as he wished, but this practice was forbidden for the wife! Athenaeus, a writer in the second century AD, quotes from a speech of Demosthenes,

“We keep mistresses for pleasure, concubines for daily concubinage, but wives we have in order to produce children legitimately and to have a trustworthy guardian of our domestic property.”

Kenneth Wuest records that

The moral life of the Greco-Roman world had sunk so low that, while protests against the prevailing corruption were never entirely wanting, fornication had long come to be regarded as a matter of moral indifference, and was indulged in without shame or scruple, not only by the mass, but by philosophers and men of distinction who in other respects led exemplary lives. (Wuest, K. S. Wuest's Word Studies from the Greek New Testament: Studies in the Vocabulary of the Greek New Testament: Grand Rapids: Eerdmans)

In the book of Acts the early church condemned all sexual experimentation outside of marriage James declaring that the Gentiles who were turning to God from idols be instructive

that they abstain from things contaminated by idols and from fornication (porneia - in this context the reference is to sexual sins in general but orgies that were associated with the worship of the pagan idols) and from what is strangled and from blood." (Acts 15:20)

The association of idolatry and sexual immorality as alluded to in this passage in Acts is frequently seen in many other passages of Scripture (Ex 32:4, 5, 6, 28, Nu 25:1, 2, 3,4, 1Cor 10:7, 8, Rev 2:14, 21:8).

In Romans 1 Paul clearly links idolatry with immorality writing...

"Professing to be wise, they became fools, 23 and exchanged the glory of the incorruptible God for an image in the form of corruptible man and of birds and four-footed animals and crawling creatures (idolatry). 24 Therefore God gave them over (a judicial term in Gr., used for handing over a prisoner to his sentence. When men consistently abandon God, He will abandon them) in the lusts of their hearts to impurity (a general term describing decaying matter, like the contents of a grave and in context speaks of sexual immorality), that their bodies might be dishonored among them. 25 For they exchanged the truth of God for a lie (an idol is a lie, a false representation of God), and worshiped and served the creature (idolatry) rather than the Creator, who is blessed forever. Amen. 26 For this reason God gave them over (God's judicial sentence because of their idolatry was to deliverer them over to the power of) to degrading (dishonoring, disgracing, shameful) passions (which resulted in erotic activity with members of their own sex); for their women exchanged the natural function for that which is unnatural, 27 and in the same way also the men abandoned the natural function of the woman and burned in their desire toward one another, men with men committing indecent acts and receiving in their own persons the due penalty of their error."  (Romans 1:22-27)(see notes beginning with Ro 1:22)

Remember that in Paul’s day the ancient pagan world in general and Corinth in particular was like much of modern day post-Christian, anti-God, relative values ruled America is today, for people were strongly intent on having their own ways and doing what was right in their own eyes (Jdg 21:25). And what they did that seemed "right" to them was particularly manifest in fulfilling their lust in illicit sexual activities, even under the guise of "religious worship.". Corinth in fact was so conspicuous for its practice of sexual immorality that to corinthianize meant to participate in reckless debauchery. Given this culture of rampant sexual permissiveness the body of Christ was sorely affected (pun intended). And so Paul writing to the Corinthian church declares that

It is actually reported that there is immorality (porneia) among you, and immorality (porneia) of such a kind as does not exist even among the Gentiles, that someone has (present tense = an ongoing, habitual activity) his father's wife."  (1Cor 5:1 read the entire chapter [1Co 5:2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13] which describes immorality in the church - and as you read be sure to observe that the admonition and rebuke in this chapter is directed not so much to those committing immorality but to the church who stood silently aloof, in fact arrogantly refusing to do anything about this horrible blemish on their local body!)

Later in this same letter Paul gives his instruction to individual believers who were being tempted to play in the sewers of sexual immorality that was so prevalent. Paul's advice was crystal clear...

What Paul is saying in the preceding passage is that sexual sin not only is against God (Ge 39:9, Ps 51:4) and other persons (1Thes 4:6-note), it is also against ourselves. Part of our moral responsibility to ourselves is to be sexually pure. When Christians are immoral, the testimony of the gospel is polluted and the holy Name of God is blasphemed. Furthermore, we can know all the truth in the world, and yet by our falling into illicit sexual activity, we are showing by that activity that we despise not only God (2Sa 12:10) but also His Word (2Sa 12:14)!

Don't dabble with porneia ("pornography"), trifle with it (there is no such thing as "soft porn" - it is a lie!), argue about it, debate it, explain it and certainly don't try to rationalize it as a "spiritual challenge" to be met but as a "spiritual trap" to be escaped. Get away as fast as you can! God gives such a clear and strong command because porneia is so serious, so seductive, so corrupting and so shattering to spiritual relationships, both human and divine! So flee for your life.

An excellent well known illustration of "fleeing immorality" is found in the account of Joseph when he was tempted to sin by Potiphar’s wife Joseph addressing her advances declared

“There is no one greater in this house than I, and he has withheld nothing from me except you, because you are his wife. How then could I do this great evil, and sin against God? [Joseph feared Jehovah and so turned away from evil, cf Job 1:1, Ps 119:104, 128, 168, Pr 8:13, 16:6]”10 And it came about as she spoke to Joseph day after day, that he did not listen to her to lie beside her, or be with her.11 Now it happened one day (God allowed this test - in every test of our hearts there is the potential that it will turn into a temptation and we will succumb to the lust of our flesh, James 1:14) that he went into the house to do his work, and none of the men of the household was there inside.12 And she caught him by his garment, saying, “Lie with me!” And he left his garment in her hand and fled, and went outside.) (Genesis 39:9-12)

While there may be safety in numbers, sometimes there is more safety in flight! It is like the pastor who cautioned his handsome new assistant about the dangers of immorality in the ministry. The assistant said that he always did his socializing in a group setting and concluded that “there is safety in numbers.” The wise pastor replied, “Yes, that is so, but there is more safety in Exodus!”

How serious is immorality? Paul's rhetorical question indicates the consequences can be eternally serious asking...

Or do you not know that the unrighteous shall not inherit the kingdom of God? Do not be deceived; neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor homosexuals, nor thieves, nor the covetous, nor drunkards, nor revilers, nor swindlers, shall inherit the kingdom of God. (1Cor  6:9-10)

William MacDonald commenting on the passage in first Corinthians adds that Paul...

does not mean to imply that Christians can practice such sins and be lost, but rather he is saying that people who practice such sins are not Christians. (MacDonald, W & Farstad, A. Believer's Bible Commentary: Thomas Nelson or Logos) (Comment: Don't misinterpret what he is saying -- he does not say Christians can never fall into this trap of sexual sin. When a Christian commits sexual sin, he or she is one of the more miserable individuals you will ever meet. And although they may be trapped by the cords of sin as in Pr 5:22, their heart's desire to not to commit the sin but to come out of that sin. If they have no desire and make no efforts whatsoever to be extirpated from that sin, that is when one has to begin to wonder whether they are truly regenerate.)

Paul goes on to explain that a healthy sexual relationship within the bond of marriage is one of the main (if not the main) defense against sexual immorality...

1Corinthians 7:2  But because of immoralities (porneia - we might paraphrase this in terms appropriate to our modern culture and say "because of the seductive danger, prevalence and supposed privacy [cp Pr 5:21, 15:3] of internet pornography"), let each man have (command, not a suggestion) his own wife, and let each woman have her own husband.

Solomon echoes this truth in Proverbs 5 calling on married men to...

Proverbs 5:15 Drink water from your own cistern, And fresh water from your own well.
16 Should your springs be dispersed abroad, Streams of water in the streets?
17 Let them be yours alone, And not for strangers with you.
18 Let your fountain be blessed, And rejoice in the wife of your youth.
19 As a loving hind and a graceful doe, Let her breasts satisfy you at all times; Be exhilarated always with her love.
20 For why should you, my son, be exhilarated with an adulteress, And embrace the bosom of a foreigner? (See notes Proverbs 5:15-23)

that everyone who divorces his wife, except for the cause of unchastity (porneia - fornication), makes her commit adultery (moichao = verb related to moichea); and whoever marries a divorced woman commits adultery (moichao = verb related to moichea) (see note Matthew 5:32)

Porneia as used in this verse in the context of marriage always constituted adultery, which, by definition, is illicit sex by a married person.

In the ancient world sexual activity (physical unfaithfulness) was often connected with the idolatrous worship of false gods (spiritual unfaithfulness) (cf Nu 25:1, 2, 3, Rev 2:14-note, Rev 2:20-note). 

The verb form of porneia is used by Paul to describe the immorality for which 23,000 (of the total of 24,000) Israelites were killed by a plague in one day ("Nor let us act immorally [porneuo], as some of them did, and twenty-three thousand fell in one day.1Cor 10:8; cf. Nu 25:9). Because the majority, if not all, of those slain were probably married, porneia clearly includes adultery.

Porneia is used most often with its lurid literal meaning but some NT uses (as well as uses in the Septuagint) utilize porneia in a figurative sense to picture idolatry or the forsaking of the true and living God to worship dead idols. Scripture describes God as married to His Church through Christ, so that any idolatry is unfaithfulness toward God and is often depicted as analogous to sexual unfaithfulness to one’s marriage partner! As alluded to earlier, this association between "spiritual adultery" and idolatry is not at all surprising in view of the fact that literal adultery was closely associated with the Gentiles, which by definition did not know God (they knew about Him but did not really know Him) and so lived in lustful passions (1Th 4:5-note). To repeat one of the best "defenses" against falling into sexual immorality is to know God.

In the OT God's relationship to Israel was pictured as that of a Husband to His wife (cf Isaiah 54:5). And so in Jeremiah we see Jehovah describe His punishment of the adulterous northern kingdom of Israel by allowing her to be defeated and exiled into Assyria (He gave "her a writ of divorce"). In spite of God's clear warning, the southern kingdom described as Israel's

treacherous sister Judah did not fear; but she went and was a harlot (Lxx = porneuo verbal root of porneia) also (why did she play the harlot? no reverential fear of the holy God). And it came about because of the lightness of her harlotry (Hebrew = zenut refers to sexual sin that violates the marriage covenant; and is used most often figuratively to describe the wickedness of the nation of Israel, this wickedness usually being associated with the worship of idols, and occasionally even descriptive of outright rebellion Nu 14:33; Lxx = porneia), that she polluted the land and committed adultery (Lxx = moicheuo verbal form of moicheia) with stones and trees." (Jer 3:8-9) 

Addressing Jerusalem, Jehovah declares to her that

you trusted in your beauty and played the harlot (Hebrew = zanah = fornicate, prostitute, figuratively of Israel's illicit relationship with other so-called gods; Lxx = porneuo verbal root of porneia) because of your fame, and you poured out your harlotries (Lxx = porneia), that she polluted the land and committed adultery (Lxx = moicheuo verbal form of moicheia) on every passer-by who might be willing...and besides all your abominations and harlotries (Lxx = porneia) you did not remember the days of your youth, when you were naked and bare and squirming in your blood." 23 "Then it came about after all your wickedness ('Woe, woe to you!' declares the Lord GOD), 24 that you built yourself a shrine (Lxx says "a house of fornication" or for harlots) and made yourself a high place in every square. 25 "You built yourself a high place at the top of every street (Lxx has "on the head of every way thou didst set up thy fornications [porneia]") and made your beauty abominable; and you spread your legs to every passer-by to multiply your harlotry (Lxx = porneia). 26 "You also played the harlot ("go a whoring" Lxx = related verb ekporneuo = to be utterly unchaste, to give self over to fornication; to indulge in flagrant immorality) with the Egyptians, your lustful neighbors, and multiplied your harlotry (Lxx = related verb ekporneuo) to make Me angry....32 "You adulteress (Lxx  = moichao) wife, who takes strangers instead of her husband!" (Ezekiel 16:15,22-26, 32 read all of chapter 16 to see how God feels about porneia!)

SEX IS LIKE
A RIVER

Sex is a gift of God which is like a great river flowing through life which, kept within its banks, is a source of pleasure and power. However when a river overflows its banks it becomes destructive and disaster ensues. In an analogous way, the water contained within bounds and channeled through the great Hoover Dam is the source of tremendous electrical power. However, if cracks were allowed to develop in the dam and allow to grow, eventually the dam would burst. Sexual impurity, is similar to a river overflowing its banks or the cracks in a dam. It begins with just a few thoughts (that come from a few "innocent" images) and grows and festers and is fed more thoughts (images) until one day it suddenly breaks down. Sexual immorality is like that. No believer commits sexual immorality in a moment. It is a gradual erosion of the believer's core values, a continual ignoring of one's conscience and a repeated repression of the indwelling Spirit's urgings to refrain, to flee, etc. As godly men and women, we must recognize the deceitfulness of any sin (Heb 3:13) but especially the particularly pleasurable (passing Heb 11:25) sin of illicit sexual activity. When the tempting thoughts come, we must take them captive by grace and the power of the indwelling Spirit and hand them over to the Captain of the hosts, our Lord Jesus Christ. (see 2Cor 10:5-). We flee from anything, anyone or anywhere, that improper sensual suggestive images are portrayed (1Cor 6:18, cp Joseph in Ge 39:12)

Within marriage, sex is beautiful, fulfilling, creative and has protective effect against immorality (1Co 7:2, 5). Outside marriage, sex is ugly, destructive, and like fire. In a fireplace, fire is warm and comforting. Outside the hearth, fire is destructive and uncontrollable.

Solomon presented a similar fiery metaphor in asking the rhetorical questions...

Can a man take fire in his bosom (Ans? Yes if he is a fool!) and his clothes not be burned? (Ans? No)
Or can a man walk on hot coals, and his feet not be scorched? (See notes Pr 6:27,28)

As Billy Graham has observed...

Satan fails to speak of the remorse, the futility, the loneliness, and the spiritual devastation which go hand in hand with immorality.

Jesus explained porneia declaring that

the things that proceed out of the mouth come from the heart, and those defile the man. For out of the heart come evil thoughts, murders, adulteries, fornications (porneia), thefts, false witness, slanders. (Mt 15:18-19)

John records that in the last years just prior to our Lord's return indescribable sexual perversions will be running rampant. He writes that those who dwell upon the earth

did not repent of their murders nor of their sorceries nor of their immorality (porneia) nor of their thefts. (Rev 9:21)

Porneia is an all-encompassing sensual or sexual immorality, a perfect description of modern day America. Let's be honest. Most men (even Christian men!) have problems with this area that they would not even dare tell anyone! When you realize that you are complete in Christ and can now say "no" to this sin, from that point on you are responsible what kind of mess you get yourself into by not saying "Yes" to Jesus and "No" to immorality. Remember that "victory" is not so much me overcoming the problem but it's me having been overcome by Christ and it's now Christ in me who can overcome. You don't have to live the way you did when you were dead in your trespasses and sin. Your body is now dead to sin (the power of sin) and alive to God in Christ Jesus.

Immorality is evidence of turning your back on God as (see notes Romans 1:25; Romans 1:26; Romans 1:27) teaches. God will judge illicit sexual sin (Heb 13:4) whether one in a believer or an unbeliever and Peter adds that but judgment begins at the household of God (see note 1 Peter 4:17) because believers are even more accountable in view of the fact that they have the power (see notes Romans 6:11, Ro 8:13, Gal 5:24) to flee youthful lusts and to abstain from fleshly lusts (1Pet 2:11). As Paul explained to believers who thought that now that they were "covered" by grace and could sin with impunity since grace abounded where sin increased, he countered this deceptive teaching with horror

May it never be! How shall we who died (dead men are positionally uninfluenced and unaffected pleasures of this life) to (the) sin (the power of sin inherited from Adam is broken along with the previous powerlessness to say "no" to it's reign and demands to be gratified) still live in it? (see note Romans 6:2)

When we surrendered to Christ and took him as Lord and Saviour, not as our "fire insurance" policy!

The biblical view of immorality is summarized in (see note 1Thessalonians 4:3) where Paul records that

this is the will of God, your sanctification (State of being set apart from sin and the world to deity God); that is, that you abstain (literally continually or as the habit of your life - present tense-  to hold oneself away) from sexual immorality (porneia).

Vine comments that Paul

puts at the beginning of the list the sins which set at defiance the primal laws of God which govern the continuation of the human race and are essential to its well-being, physical and moral...The first in this list is a specific sin; from this there is a transition to the moral general. (Vine, W. Collected Writings of W. E. Vine. Nashville: Thomas Nelson)

William Barclay has an interesting historical note to put Paul's teaching in the proper cultural context writing that

Chastity was the one completely new virtue which Christianity brought into the world. In the ancient world sexual relationships before marriage and outside marriage were the normal and accepted practice. The sexual appetite was regarded as a thing to be gratified, not to be controlled. That is an attitude which is not unfamiliar today, although often it is supported by specious arguments. The Christian ethic insists on chastity, regarding the physical relationship between the sexes as something so precious that indiscriminate use of it in the end spoils it." (Barclay, W: The Daily Study Bible Series, Rev. ed. Philadelphia: The Westminster Press)

Paul has another "vice" list in Galatians writing that

the deeds of the flesh are evident, which are: immorality (porneia), impurity, sensuality, idolatry, sorcery, enmities, strife, jealousy, outbursts of anger, disputes, dissensions, factions, envying, drunkenness, carousing, and things like these, of which I forewarn you just as I have forewarned you that those who practice such things shall not inherit the kingdom of God. (see notes Galatians 5:19; 20; 21).

John records a similar warning writing that

for the cowardly and unbelieving and abominable and murderers and immoral persons (related noun pornos) and sorcerers and idolaters and all liars, their part will be in the lake that burns with fire and brimstone, which is the second death.  (see note Revelation 21:8)

Neither Paul nor John is not saying that you won't go to heaven if you slip up and commit these sins from time to time. But they do indicate that if these sins are your lifestyle (and if there has never been a time in your life when you experienced a degree of freedom from them) then you will inherit not the kingdom of God but the lake of fire.

The Bible Knowledge Commentary adds this insight on Revelation 21:8...

"It should be obvious that this passage is not affirming salvation by works, but rather is referring to works as indicative of whether one is saved or not. Obviously many will be in heaven who before their conversions were indeed guilty of these sins but who turned from them in the day of grace in trusting Christ as their Savior. Though works are the evidence of salvation or lack of it, they are never the basis or ground of it." (Walvoord, J. F., Zuck, R. B., & Dallas Theological Seminary. The Bible knowledge commentary : An Exposition of the Scriptures. Wheaton, IL: Victor Books)

Someone has written that immorality is like a cat’s paw. When lightly stroked, it is quite soft and pleasurable, but increased pressure brings out the claws of sin that will shred your very life.

Pastor Ray Stedman writes that

Words like immorality do not seem to register with many people. Let us put it plainly: Immorality means no sexual wrongdoing; no pre-marital sex (no fornication); no extra-marital sex (no messing around with someone else's wife or being faithless to your own husband or wife); No homosexual sex (that is very clear in Scripture in many places); No pornography (no standing in the news section at the airport and flipping through Penthouse or Playboy magazine and getting yourself turned on by looking at the pictures; that is sexual fantasy and that is wrong, too, as Jesus pointed out). So to "flee immorality" means to have none of those things going on in your life. (from Handling your Sex Drive)

Miscellaneous related resources from John Piper:

Strategies for fighting sexual sin, 
ANTHEM - Strategies for Fighting Lusts;
10 Steps Toward Sexual Sin: Pitfalls and Protection for Christian Leaders

If you know someone "wrestling" and being continually defeated by Internet Immorality (and they want to be set free) have them check Covenant Eyes and then agree to be their accountability partner. Under grace and with the Holy Spirit's enablement (Gal 5:15, Ro 8:13), I have heard a number of men give testimony of the efficacy of this tool. The difference between Covenant Eyes and other "porneia" blocking software is that Covenant Eyes logs every place you visit and keeps that record on a separate computer at their headquarters. Your site visits are scored as to likelihood that they have porneia and your accountability partner receives a periodic report. Men need accountability in this day of widespread, seemingly "anonymous" access to internet pornography. Remember though my beloved brethren in Christ

The eyes of the LORD are in every place, watching the evil and the good. (Pr 15:3)

Thus although there may be