Mostly because any bad genes are doubled. Haven't yet seen a gay union produce an offspring have you??Me too.
I don't buy that logic.
Sex between a mother and a son, and between a brother and sister, should also be fine as long as it's between consenting adults, going by that logic.
If we allow gay sex, why not sibling sex?
Humans have bad feelings about such relationships because very early in their history, they saw the results of such unions. They soon found a new gene pool produced a much smarter and healthier person. Both parents carrying a nasty recessive gene produces a dominant one. Probably that is how a lot of particular diseases exist within certain races or gene pools. .