I don't think it's genetic, but I don't think it's a choice, either. I'm not a biologist, so I'm not too sure what could possibly cause it; however, it most definitely is natural, as there are many cases in which animals have displayed homoloveual traits. Apparently, a case with rams show that the homoloveual rams had smaller genitalia than the heteroloveual ones, so I am certain that hormones do have some sort of effect on one's loveuality.