The original set-up of humans was polygnyous while humanity lived in tribes. The leader - the alpha male - would father the majority of the children and they were raised collectively (alloparenting). When we implemented civilisation with intense labour, that's when monogamy became the norm.

It's also why only 40% of men in history have fathered children compared to 90% of women.

As an aside, modern day dating and social media is starting to mark a return to polygnyny and the state taking over the role of father is encouraging alloparenting to return too where women collectively raise children from the same man paid for by the state (and in turn, all the men who can't find a partner and have a family). I've even started to see men have multiple women on the go to support them and collective raising of children between women. Just as it was in tribal times.

It's rather interesting to see conservatives attempt to justify this in complete contradiction to the family values they promote. Normally polygnyny is promoted by the left.