Nothing you have said in any way contradicts what I said. In many cultures the cheater of either sex who is found out is ejected from their original relationship and faces dire social consequences. In some impoverished cultures two men, often brothers, will share a wife to increase the family chances of success.

Humans are adaptive and inventive, their family formation solutions change with the challenges they face, and in many cases the monogamous pair bond is a powerful and successful model.