this is a knee-slapper.  and a head-scratcher.
if they don't participate in evolution, why don't they all still look like Cro-Magnon?
So Bean, how's this - - suppose a male homosexual has sexual relations one time with a woman.  Then he is no longer a homosexual, right?  So then he could marry another man, right?
Or maybe he can just go to a sperm bank and make a deposit. Then he's not a homosexual anymore because he's contributed to genetic diversity.  
Sorry to all you lesbians out there though, I guess your solution isn't quite as easy
OK, I'm sounding really dumb now.