Evolutionarily speaking does homosexuality serve a purpose? If it didn't it's hard to imagine it persisting in the gene pool.

I find it hard to believe sexuality could be determined or influenced by upbringing because if that were the case animals wouldn't be as overwhelmingly heterosexual as they demonstrably are.

So is the gay just the result of random mutations in the gene(s) that control sexuality? It can't be as simple as flipping on "female mode" in the sexuality gene because not every gay is an effeminate bottom.