Marrying a divorced woman with a kid and supporting that kid in any way is polygamy. Having a kid with that mom that has a kid with someone else is polygamy. Evolution doesn’t care about whether she is divorced or not. Evolutionary mechanisms will kick in the same, whether it is an african woman who has multiple children by multiple men or a white woman who does the same but gets a divorce in between men. Its matrilineal polygamy. When men cannot be assured of the paternity of the children they are devoting resources to, the only reproductively successful men are the ones that dont stick around. Let me make this clear. Where ever females are found to commonly have more than one sex partner, the society does not recognize an obligation of dads to take care of their children. This is the african model of family— mom lives with brother and grandmother. Dad does not hang around. Evolution disincentivizes it. It seems that it is such a penalty to raise a child that does not belong to you that men that do it go extinct. Only where females have a single sex partner do you see fathers investing considerable resources in children. Certain genes are favored in polygamy, other genes are favored in monogamy. Do you understand why I say it doesn’t matter that you think this isn’t polygamy? Nature thinks its polygamy.