Pornography
in The word of God,
Matthew 5:28 "If a man looks on a woman to lust after her, he has committed adultery with her already in his heart."
Psalm 101:3 "I will set no unclean thing before my eyes."
Philippians 4:7, 8 "Keep your hearts and minds in Christ Jesus... Whatsover things are pure... if there be any virtue... think on these things."
The word "pornography" comes from the Greek porneia, porneuo, and porne. Porneia is translated "fornication" in the King James Version (Mat 5:32; John 8:41; Rom 1:29; 1 Corin 6:9,13,18; Gal 5:19; Rev 19:2; etc.). Porneuo is the verb "to commit fornication," as in 1 Corin 6:18. Porne is translated "harlot" in Mat 21:31-32; 1 Corin 6:15; and Rev 17:5. 
Whoremonger
Ephesians 5:5 "For this ye know, that no whoremonger, nor unclean person, nor covetous man, who is an idolater, hath any inheritance in the kingdom of Christ and of God."
1 Timothy 1:10 "For whoremongers, for them that defile themselves with mankind, for menstealers, for liars, for perjured persons, and if there be any other thing that is contrary to sound doctrine"
Hebrews 13:4 "Marriage is honourable in all, and the bed undefiled: but whoremongers and adulterers God will judge."
Revelation 21:8 "But the fearful, and unbelieving, and the abominable, and murderers, and whoremongers, and sorcerers, and idolaters, and all liars, shall have their part in the lake which burneth with fire and brimstone: which is the second death."
Revelation 22:15 "For without are dog, and sorcerers, and
whoremongers, and murderers, and idolaters, and whosoever loveth and maketh a lie."
Many lustful sins take place in the dark, in secret, but nothing is hid from God:
Hebrews 4:13 "All things are naked and opened to the eyes of Him with whom we have to do."
Numbers 32:23 "Be sure your sin will find you out."
More serious than other sins?
Pornography seems more disgusting than other evils.
1 Corinthians 6:18 "Every sin that a man doeth is without the body; but he that committeth fornication sinneth against his own body. "
1 Corinthians 6:18-20 "What? Know ye not that your body is the temple of the Holy Ghost which is in you ...? therefore glorify God in your body, and in your spirit, which are God's."
'Just looking'?
Some would argue that looking at the naked human body is not evil, because God made it beautiful, and Adam and Eve were naked in the garden. However, that was before sin entered the picture.
"The eye is the window of the soul," as the saying goes. What we look at does affect us spiritually - and can cause us to sin. David "SAW" the woman naked, before he got ideas to sin with her. Jesus said that if a man "looks at a woman to lust after her, he has committed adultery with her already in his heart"; etc. The sin takes place even before the action is executed.
Scripture gives many examples of men being enticed by the sight of a woman. Shechem saw the woman before he raped her. David saw the woman bathing before he had sex with her. Judah saw the harlot before he had sex with her. It was the same for Samson, etc.
Genesis 34:2 "And when Shechem the son of Hamor the Hivite, the prince of the land, SAW HER, he took her and lay with her by force."
2 Samuel 11:2-5 "Now when evening came David arose from his bed and walked around on the roof of the king's house, and from the roof he SAW A WOMAN BATHING; and the woman was very beautiful in appearance. So David sent and inquired about the woman. And one said, 'Is this not ... the wife of Uriah the Hittite?' And David sent messengers and took her, and when she came to him, he lay with her... And the woman conceived; and she sent and told David, and said, 'I am pregnant.'"
Matthew 5:28 "but I say to you, that everyone who LOOKS on a woman to LUST for her has committed adultery with her already in his heart.
Genesis 38:2-3 "And Judah SAW there a daughter of a certain Canaanite whose name was Shua; and he took her and went in to her. So she conceived and bore a son and he named him Er...."
Genesis 38:15-16 "When Judah SAW HER, he thought she was a harlot.... So he turned aside to her by the road, and said, 'Here now, let me come in to you'; ...And she said, 'What will you give me, that you may come in to me?'"
Judges 16:1 "Now Samson went to Gaza and SAW a harlot there, and went in to her."
Judges 14:2-3 "So he came back and told his father and mother, 'I SAW a woman in Timnah, one of the daughters of the Philistines; now therefore, get her for me as a wife.' Then his father and his mother said to him, 'Is there no woman among the daughters of your relatives, or among all our people, that you go to take a wife from the uncircumcised Philistines?' But Samson said to his father, 'Get her for me, for SHE LOOKS GOOD TO ME.'"
Visual stimulation is more common in man than in woman, however. There's just one example in Scripture of a woman lusting after a man by SEEING him:
Genesis 39:7-12 "And it came about after these events that his master's wife looked with desire at Joseph, and she said, 'Lie with me.' But he refused and said to his master's wife, 'Behold, with me here, my master does not concern himself with anything in the house, and he has put all that he owns in my charge. 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?' 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. Now it happened one day that he went into the house to do his work, and none of the men of the household was there inside. And she caught him by his garment, saying, 'Lie with me!' And he left his garment in her hand and fled, and went outside."
How can addiction to porn be overcome?
Giving in to lustful desires is a sign of spiritual weakness. Ephesians 6 tells us what armor to put on, to be "strong" in spiritual battle.
If you don't want to get slaughtered in battle, don't go dancing in the minefields. Former alcoholics don't hang out in bars. Fill that free time with a positive activity: Read a book. Sing a psalm. Call a friend. Visit a nursing home. Pray. As the saying goes, "Idleness is the devil's workshop."
Christians especially should not practice such sins, as we will be held to a stricter standard:
1 Peter 4:17 "Judgment must begin first at the household of God."
Luke 12:48 "To whom much (light) has been given, much (responsibility) will be required."
Is it possible to overcome this sin?
1 Corinthians 6:9, 11 "...such were (past tense) some of you. But now...."
Being tempted is not sin (even Jesus was tempted) - but giving in to it, or seeking it out, surely is.