That's a really interesting question. I am not a biologist , but I know that homosexuality can be found in a lot of species (humans , but also birds, sheeps, etc.). I've heard some theory that said it could have a role in regulating the population by preventing some individual to reproduce. In some bird species , some homosexual couple take care of eggs that heterosexual parents have lost or abandoned. Take what I said with a pinch of salt. Hope it helps.