From "Why is Polygamy Declining?" by Matt Ridely:
So at some point in the distant past, we developed the habit of monogamous pair bonding. Intellectuals, from Rousseau to Engels to Margaret Mead, have been tempted to speculate about a promiscuous human past not so long ago, from which marriage crystallised. Initial encounters with other civilisations based around agriculture and full of polygamy, such as in Mexico or Tahiti, at first seemed to confirm this idea, but when in the 20th century anthropologists began getting to know hunter-gatherers (supposedly the most primitive level of society), they were startled to find that monogamous marriage predominated in them. In human beings, monogamy probably goes back hundreds of thousands if not millions of years.
Polygamy, in this reading, was mainly an aberration of the last 10,000 years caused by agriculture, which allowed the accumulation of huge surpluses, which powerful men translated into prodigious sexual rewards. Herding societies in particular became highly polygamous, causing people with names such as Attila, Ghenghis or Tamerlane to conquer other lands so as to supply women to their sex-starved followers: polygamy and violence tend to go together.
However, the winners from a polygamous system are not just the high-status men, but also the low-status women. The peasant girl who joined the palace harem achieved safety, plentiful food and access to luxuries, while her brother languished in celibate poverty. The losers are the low-status men and the high-status women.
It makes evolutionary sense that high-status males are attractive to women, because they were in the past likely to be able to ensure the success of any children they fathered, and that men are attracted to what Amazonian Indians call “moko dude” women. (The phrase means “ripe” when used of fruit and, when used of women: “of the right age, health, genetic quality and unencumberedness likely to make them capable of producing many healthy children and grandchildren”, or, more pithily, “phwoar”.)
So how come the president of France, with the status of a monarch, cannot even get away with two women at a time? Inch by inch, from Odysseus to Figaro to Bill Clinton, Western mores have insisted on monogamy even for the powerful. Clearly the interests of high-status men and low-status women have lost out to the interests of high-status women and low-status men.