All true, but you fall into that individual genetic trap. :idea:
1) There is no necessity that the Gay gene is allocated at the Gay individual himself (it's in the mother, most probably); and
2) For the evolution, decisive are population genetics - means the tribe survival is more important for the gene than survival of any particular individual.
In our particular case, the biology is apparently determined in the mother's womb - the more brothers you had before you, the more chances you have to become a pretty little baby-Gay. So it's your mother's genes which are responsible for your body, not your own genes - amazing! For the tribe, Gay people are of greate advantage - they strengthen the social network and pick up spiritual/social duties. Or at least it was so, before certain religions were invented - and the humankind tore apart. From respected family members, Gays turned into scapegoats and bugbears to scare little and big boys.