I love how you think about evolution as competing pulls. When you have two adaptive traits that contradict each other, it can result in a a sort of balanced stasis.

I do question the assumption that monogamy is adaptive over the last million years. My understanding is that before agriculture and the invention of money, the accumulation of wealth was a non-issue. If people lived in a semi-nomadic group and food was secured, it made sense to share that food across the whole group. This means that likely the evolutionary unit was the whole band and not the nuclear family. In that scenario, promiscuity would be favored for both women and men. This theory is a condensed version of the Sex at Dawn book and might also be found in Wednesday Martin's Untrue (if I ever picked it off the pile and read it).

The theory goes that with the advent of agriculture (12,000 years ago?) food could be amassed, stored, and transported for the first time. This allowed the "strongest" man to control large amounts of wealth and changed the economics from cooperation to competition, both for wealth and for mates. This leads to men trying to monopolize the reproduction of women and using the safety that wealth provides in order to cement the deal.

Kim Tallbear talks about monogamous marriage as a tool of the settler-colonial state, and I'm inclined to agree.

So we have at least 300,000 years of human evolution in hunter/gatherer bands that tended to be promiscuous, followed by 12,000 years of agriculture-induced patriarchy. That is the evolutionary push/pull that I see, which ends up being the same one you see, i suppose. Although it is possible that monogamy is entirely cultural and not evolved, which is why it fails so often and needs so much propping up.

As an aside, The Dawn of Everything authors are blowing big holes in the conventional wisdom of the steps of progression of society from egalitarian hunter/gatherer bands into modern day patriarchal cities. They say that smooth progression is not supported by the archeological evidence. That is a hopeful look because it proposes that the push-pull is still in action (in their case the push and pull between cooperation and competition - but it easily translates into monogamy and promiscuity).

Regarding your idea for normalizing sex work, I am all in favor.