Now and then people experiment with changes in basic social arrangements. They may institute polygamy, or start marrying their first cousins, or worse yet their sisters. They may, as a society, decide that their brightest young women should waste their reproductive years on careers. Anyone who understood much about genetics would know that those experiments were bound to have bad consequences, but back in the past, nobody knew anything about genetics, with the possible exception of some farmers. Of course nobody knows anything it today, either. In fact, no popular ideology is at all based on any kind of genetic knowledge or understanding, and some strongly oppose the actual facts on the ground. Others simply don’t give a rat’s ass about the long-term consequences.
But you can’t beat the facts. The advanced paternal age associated with polygamy increases genetic load. Cousin marriage increases infant mortality and decreases IQ. The Hapsburgs turned into a mess, the Ptolemies went straight downhill, and King Tut had more things wrong with him than you could shake a stick at.
History shows again and again how nature points out the folly of men.