For adultery (sex with any one besides your spouce (and touching yourself could be considered sex with yourself))
1st Corintians 6:9
1st Timothy 1:10
Exodus 20:14
Dueteronimy 5:18
Matthew 5:27, 5:28, 5:32, 15:19, 19:9, 19:18
Mark 7:21, 10:11, 10:12, 10:19
Luke 16:18, 18:20
John 8:4
Hebrews 13:4 "Marriage should be honored by all, and the marriage bed kept pure, for God will judge the adulterer and all the sexually immoral." Note that last part is seperate from adultery and refers to something more.
All this just for starters. I don't have time to go through every book and scroung out all the verses.
Now, for more on sexual immorality and what I was saying also:
1st Corinthians 6:12-20 "'Everything is permissible for me'--but not everything is beneficial. 'Everything is permissible for me'--but I will not be mastered by anything. 'Food for the stomach and the stomach for food'--but God will destroy them both. The body is not meant for sexual immorality, but for the Lord, and the Lord for the body. By his power God raised the Lord from the dead, and he will raise us also. Do you not know that your bodies are members of Christ himself? Shall I then take the members of Christ and unite them with a prostitute? Never! Do you not know that he who unites himself with a prostitute is one with her in body? For it is said, 'The two will become one flesh.' But he who unites himself with the Lord is one with him in spirit.
Flee from sexual immorality. All other sins a man commits are outside his body, but he who sins sexually sins against his own body. Do you not know that your body is a temple of the Holy Spirit, who is in you, whom you have recieved from God? You are not your own; you were bought at a price. Therefore honor God with your body." [itallics not in original; added for emphasis]
1st Corinthians 10:8 (lessons from the history of the jewish people) "We should not commit sexual immorality, as some of them did - and in one day twenty-three thousand of them died."
Galatians 5:19-21
Ephesians 5:3
Collosians 3:5
1st Thessalonians 4:3
Again, for starters.
And again, Paladin is totally right, that the issue, as with others that may be argued as to be matters of what is personally allowed within a person's level of Faith, must be regarded as a sin in any case where it makes a brother or sister in Christ stumble in regards to their own faith. Not to mention, that the case that it's an absolute sin is incredibly steep; and I would guess the only reason it's not there by direct name is that it isn't in the language of the time like that, and indeed is covered by sexual immorality, as all sex outside of marriage (before marriage, as after marriage it becomes adultery) is considered such.
Edit: Let us not get into legalism and the horrid trap it is! But let us not be fools either and try to justify what is wrong; as that is just as great an ill and will lead to downfall whatever the case. It is an issue many struggle with, so no wonder it is so strongly argued on all sides, but is it so hard to have selfcontrol? I fail at times, but I won't stop fighting and thus I grow stronger. We are being sanctified by the Lord in this life, is it HOLY to do these acts? Ask yourself that; as salvation and subsequent sanctification is about holiness and righteousness and purity before the Lord.
And what image do we display to the world around us? Do we display something that will bring people to Christ, or only make them comment about how we are the same in corruption as everyone else?