There you go again. Define unnatural.There is no reasonable reason that homosexuality exists in humans. It's a fetish. That doesn't mean, in and of itself, anything though. We do plenty of unnatural things that are completely accepted.
"Never fear. Him is here" - Captain Chaos (Dom DeLuise), Cannonball Run
Mace Windu: Then our worst fears have been realized. We must move quickly if the Jedi Order is to survive.
What purpose is served by heterosexual sex that isn't for the sole purpose of reproduction? Why are other parts of the body sexual arousal points, including the prostate when going through the anus? Why do people who are born infertile feel any sexual arousal? Why do women still want sex after menopause?
Many of the answers that go with these questions could go for why sex other than intercourse, could serve a purpose or multiple purposes.
And homosexuality alone could serve the purpose of buffering population growth and/or providing a suitable parental substitute in the absence of biological parents. Either of these things would help a species. Slowing the rate of population growth would be helpful to ensure resources are not depleted at a rate that would kill off the entire species to the point of extinction before the resources can replenish themselves. There are other things in place, but homosexuality could reasonably be one of those buffers. (Nature would not take into account human efforts to control or limit homosexuality through use of authority. Neither would nature take into account human ability to artificially procreate, without sex.)
These are all just speculation, but so is your own belief that homosexuality serves no purpose in nature. I am showing you that it could. Plus, it is pure speculation to believe that humans were only "meant" to have sex for procreation. I don't believe that we would have the sex drives we have if this were true, especially since most individual women can only get pregnant for about a week out of each month, yet few women just want to have sex around ovulation time.
"A woman is like a teabag, you never know how strong she is until she gets in hot water." - Eleanor Roosevelt
Keep your religion out of other people's marriages.