Homosexuality is as natural as heterosexuality. I believe it's like with your hair colour: some of us are blonde, some are dark haired; some are homosexual, some are heterosexual (you don't chose either). Some theories in fact say that we are all essentially bi-sexual.
Homosexuality has been present as far as the world goes it's just that cultures are more / less accepting.
Small traditional societies are less likely to be tolerant of homosexuality.
Now, this will probably be a risky thing to say of me, and I hope you can be open minded about it and have an educated discussion here....
I think that homosexuality is a form of random variation naturally happening without a specific reason in all species.