Although homosexual behavior exist in the animal world, it's very uncommon that individual animals have a long-lasting predisposition to engage in such behavior to the exclusion of heterosexual activities.
Geneticists, anthropologists, developmental psychologists, sociologists, endocrinologists, neuroscientists, medical researchers into gender, and twin study researchers are in broad agreement about the role of genetics in homosexuality. Genes don’t make you do it. There is no genetic determinism, and genetic influence at most is minor. Thus it's "unnatural."
Homosexuality ain't genetic at all, if it were, homosexuality wouldn't have existed as of now. It would cause it's own extinction over the ages, not to mention how strongly it contradicts the Darwinian natural selection theory. That plus the obvious fact that identical twins should supposedly share the homosexuality gene 100% of the time, this isn't true however... As there's no homosexuality gene to begin with.
And before anyone replying with: "Not everything that is biological is genetic. Sexual orientation is biological even though it is not the product of a single gene." (quoted from a homosexual supporter I last debated) This is not logical at all. If homosexuality were biological without having any genes, I can easily say that me loving Anime and JRPGs is biological without any "otaku gene" being in there.
In short: homosexuals aren't born homosexuals, but they're made homosexuals when they grew up. And as Dr. Hamer conceded (The creator of the gay gene hoax himself), homosexuality is culturally transmitted, not inherited.
How you think of homosexuality ethically is another subject. I personally don't support homosexuality, but I don't "hate" on homosexuals nor treat them differently from any other just because they believe that they're supposed to be that way.
Saw a lot of "homosexuality". It's not quit the same as saying I'm going to change my body and change my entire being. ---