I know it isn't 100%, but I was always told it is high enough, that it isn't worth the risk, because child will always have some issues and much bigger chance than average for "jackpot".
So are children form people who have genetic diseases or from women who are 40+..or who are smoking...or drinking. Yet no law forbids them to have sex and kids.
Just imagine that, a politician trying to push a law the forbids people with genetic mental illnesses to have children. The media would have a field day with that one.
Yet, why are incest relationships still forbidden? Why is it 'worse' for them to have children?