I read yesterday that homosexuality is a "genital revolution", being rebellious in doing what one wants with ones own genitalia. This suggests that it is rebellion, a social response, like being a badass.
On the other hand my personal opinion is that gays posses an innate ability to pervade social norms. Possibly the weak argument against homosexuality is spotted more easy by those who have homosexual or bi sexual relations. This would involve a difference in neurological genetics, ie maybe a difference in the frontal lobe or other problem solving areas of the brain. Just a theory though.