So by your definition homosexual behavior cannot be "natural" since it doesn't support the continuation of the species, correct?
I can't think of anyone who would consider the obsession with celibacy as something natural. This idea is sold as some form of piety only to be released upon children behind closed doors.
All religions claim legislation to regulate matter of sex and the more extreme prurience, the more extreme the repression. Sexual impulses are laden with prohibition, guilt and shame. Masturbation, oral, anal, non-missionary position. Saint Paul in the New Testament expresses both fear and contempt for women. There's some primitive fear that an entire gender is unclean and can be defiled, but is at the same time a temptation to sin that is impossible to resist. Islam uses the angle of sexual indulgence as reward in the afterlife. Christianity is far too repressed for such an offer and never has been able to form up as compelling an afterlife as islam. Instead, it offers up lavish promises of sadistic eternal suffering for sexual non-adherents, which is really the same point made in a different way.
As you're helping me think it out, I'm beginning to think religion's obsession with sexual repression in general is the root of the specific issue with homosexuality and not really the act itself.