This IS the sort of aberration that affects future children of the offspring of such incestuous behaving people; and those future children who will become adults and possibly unwittingly pair and have children with the products of such physically risky human reproduction are, or should be, a consideration. There was a REASON (beyond mere punishment) why scripture forbade such behavior, and a REASON why in history there have been LAWS against such behavior. - What if YOUR grandchildren meet and unwittingly marry the grandchildren of this poor child? Will you then still be so TOLERANT? Let me tell you from experience, even in the normal routine of things, it is a real SHOCK to have a retarded or otherwise disabled child. (Usually, too, future generations of this kind of aberration in behavior are ashamed of such and hide it from the world.)
There are many things scriptures of all types forbid, with or without laws. Last year we were invited to a church ceremony celebrating a divorce. The pastor felt it was important to the children for the couple divorcing to celebrate their good qualities before separating for the children's sake. WTF? I would of course not attend.
Of all the ills society is suffering regarding family and marriage, I say this doesn't have a rat's arss to do with the world's woes.