Well for one it's not a viable option for maintaining a population. It literally makes it impossible for two people to carry out the number one goal for a living thing, reproduction. And is homosexuality actually common in other species? I find that hard to believe. Also how the hell are you supposed to have a child if you're completely gay without any help from the opposite sex? I'm not saying it's wrong and I'm not homophobic or anything. If you love someone you should be able to be with then. It's just that it's literally biologically impossible for an entirely gay species to survive.