Originally posted by Sterling
I don't see how it can't be the same way with heterosexuality. The whole "instinct" thing won't work, either; male giraffes, foxes, penguins and over 1,500 different species of other animals **** each other.
I still think it has to do with hormones, because considering what people think about psychology pertaining to animals, we think it to be impossible. But since it's obviously not, as it occurs quite frequently, it all depends of the individual.
No one is born heterosexual, homosexual, nor any other sexuality. Sexual orientation is "developed" later in life. Otherwise, the "cooties" thing wouldn't happen between boys and girls at a young age, if we were meant to be, by genetic default, heterosexual.
If you want to be technical, it's possible that sexuality is based on genetics. Like brown hair, eye color, etc; it varies from person to person. (And don't forget, genetic doesn't always mean inherited.) But I'm not entirely sure.
EDIT: Re-reading your post, you say it's not a choice, but if the family is anti-gay, the teen will choose to be gay due to a rebellion factor. Inconsistency with your logic. =D