Because adults have the power to choose what they do and don't want to do. If it is "perverted" (whatever that means), it is of their own free will, and as long as they don't involve me in it, I don't care. Children, OTOH, have little or no voice in what happens to them. In addition, the latest, and very recent, fruit fly research indicates strongly that homosexuality in the animal kingdom (and thus, very possibly, also in humans) is caused by errant genes rather than upbringing or experience. Do a search on "fruit fly" at Google or read this article from the Independent:
Sexuality determined by nature or nurture? Fruit fly gives the answer.