"Flee from sexual immorality. All other sins a man commits are outside his body, but he who sins sexually sins against his own body." (1Corinthians 6:18)
"It is God's will that you should be sanctified: that you should avoid sexual immorality; that each of you should learn to control his own body in a way that is holy and honorable, not in passionate lust like the heathen, who do not know God; and that in this matter no one should wrong his brother or take advantage of him." (1Thessalonians 4:3-6)
"Marriage should be honored by all, and the marriage bed kept pure, for God will judge the adulterer and all the sexually immoral." (Hebrews 13:4)
Apparently, according to everystudent.com, these are all passages from the Bible that prove why, and I quote, "Several areas of sex are NOT okay with God: incest, sex with animals, sexual involvement with someone you are not married to, sex with someone who is married to another person, and homosexuality are all referred to as immoral and wrong before God. Frankly, all of our sin (not just sexual sins) are an affront to God who is holy. "
How those passages actualy support this statement I'm not quite sure. However, my favorite line from this whole page is: "Most people today determine their sexual involvement based on personal standards. This of course is fair, because on one hand, sex is a personal responsibility and must be determined personally. On the other hand, sex usually involves two people, so it's not just a matter of personal standards, is it?" Umm... usually? If we're talking actual sex I think that requires 2 people.... but that's just me.
All quotes and passages from:
How do you set your sexual standards?