They don't have to necessesarily have their own children, they can just adopt them, if that is the problem. Also, my grandparents that are cousins had 9 kids, none of them with genetical problems. And you made some projection there, I am not into any "onanistic bubble" since I was not even touching this aspect, I am not dealing with this as a fetish, but only the social problems that these kind of couples has.