I feel you are still confusing proximate and ultimate here. Maybe I'll make a new post discussing the differences. It is worth addressing. Humans tend to live in monogamous relations, but in some cultures it is common for men to have more than one wife. I take exception to your comment that modern human societies are artificial.
All human societies are natural.
The diversity of human societies, in my view, is not evidence that they are artificial deviants from some natural state. This plasticity or pliability of the mind and of resulting behavior, itself can be selected for by natural selection, making it in every sense, natural. For this reason, I do not think you can call the resulting variety of human social systems artificial. Well you are a stud who takes an interest in his body - an exception. The rest of us eat because we're hungry or because it tastes good.