alt Hacker NewsMostly monogamous marriage is almost certainly a stable equilibrium given that you see it in societies all over the world that had no contact with each other and little in common otherwise. You see monogamous marriage in neolithic europe as well as among native americans.
I suspect that before the invention of agriculture and ownership of land, serial monogamy with occasional fooling around was the most prevalent relationship style. Once people could develop wealth they started worrying about passing it down to their children and that led to more settling down.