The Bible has words that clearly describe immoral acts. Consider the Greek words pornos and porneia. Here are the definitions as taken from Strong’s Exhaustive Concordance of the Bible:
4205 pórnos (from pernao, “to sell off”) – properly, a male prostitute; in the NT, any “fornicator” (Abbott-Smith); i.e. anyone engaging in sexual immorality. See 4202 (porneia).1
4202 porneía (the root of the English terms “pornography, pornographic”; cf. 4205 /pórnos) which is derived from pernao, “to sell off”) – properly, a selling off (surrendering) of sexual purity; promiscuity of any (every) type.2
So, “selling off” our sexuality is one way to define pornography. Also, images or words which we allow to lead us to sexual immorality or promiscuity would be considered pornography. The Greeks were not the first to wrestle with nakedness and morality. We see the beginnings of this struggle in Genesis where Adam and Eve became self-conscious of their nakedness after they had sinned in the Garden of Eden. Before they sinned they were naked and comfortable. After sin entered into the world there was an awareness of their nakedness and a desire to cover themselves. This sense of nakedness is what allows immorality and pornography to develop. Sin and nakedness are not one and the same but they have been intertwined ever since.
There are certain images and situations which are clearly immoral, unjust, and absolutely qualify as pornography. Anytime a person is degraded, objectified, hurt, or disadvantaged by the process of taking the image, there is injustice and pornography involved. Some of the magazine industry in our country falls into this category. Even when a person is paid for their modeling work, the process may represent objectification and an unjust situation.
There is another factor in this struggle; it is the struggle of the mind. For the mind can turn innocent images or art into pornography. All of us must control both what we allow ourselves to see and the thoughts we allow ourselves to have. By personal will and by the work of the Holy Spirit, we seek to think upon pure and good things so that what we see in this hyper-sexual majority culture does not cause us to sin. We will never be able to avoid all possibility of seeing pornographic or sexually charged images; a short walk past the underwear displays in the mall will convince us of that. Instead, we must train our minds to stay pure even if exposed to an image we did not wish to see or an image at which we allowed ourselves to look. Perhaps the best that we can do is to always fill our minds with good things and good desires for others. This is what the Bible means when we read these words in Philippians 4:8.
And now, dear brothers and sisters, one final thing. Fix your thoughts on what is true, and honorable, and right, and pure, and lovely, and admirable. Think about things that are excellent and worthy of praise. (New Living Translation)
1 Strong’s Concordance: http://www.blueletterbible.org/lang/lexicon/lexicon.cfm?Strongs=G4205&t=KJV&ss=1
2 Strong’s Concordance: http://www.blueletterbible.org/lang/lexicon/lexicon.cfm?Strongs=G4202&t=KJV