Wrong. There are no genetic issues to deal with when speaking of homosexuals marrying.
However, allowing brothers and sisters to marry would go a long way towards eliminating the usury Estate Tax.
Society disagrees with your logic. HIV adults have kids not too mention those unfortunate individuals who carry the genes for cystic fibrosis, hemophilia, sickle cell disease, etc.
I think gay marriage, polygamy, and incest will further erode the atomic structure of our civilization to the detriment of all of us and especially children. However, there will be no legal or rigorous medical basis to prevent consenting adult sibling or parent-adult children marriages.