Well, there's at least 100,000 years of human existence, so really, wouldn't one suppose there's been ample time for all those nasty gay genes to be extinct?
Unless, oh, whoops -- wait, evolution happens at the group level, too (families do tend to share the same genes, as you might perhaps guess) and perhaps the genes stick around because a man with a gay brother has kids who have an uncle who may not want to go off to the hunt, or the raid on the neighbouring tribe, and so stays home and protects his nephews and nieces that they might just live long enough to pass on their father's genes.