My beliefs regarding sex are that it is a sacred thing that was designed by God and is thus a good thing, but like all such things it is easily abused. I believe it is intended for a married, mixed-gender couple. I believe that sexual preference is a far more fluid thing than people talk about it being (at least in terms of labeling people heterosexual, homosexual, and so on). I also believe that sex, when engaged in outside of its intended purposes (not only procreation, but also emotional/psychological bonding and, obviously, pleasure between a husband and wife) can be terribly destructive, ruining relationships, spreading venereal diseases, causing unwanted pregnancies, creating emotional bonds that result in damage when the participants don't give the proper respect and care to those bonds that they've created, and generally degrading what is a beautiful and pleasurable act into something tawdry and selfish. I believe Jesus when he said that when a man looks at a woman [that he's not married to] with lust, he's committing adultery in his heart and sinning against God.
My beliefs regarding sex mostly stem from the Bible, but are corroborated by personal experience, what my parents taught me, reading essays and reports on the subject, and from observing other people's lives and the effects sex has had on them, both positive and negative, both in obedience toward God and in defiance of Him.