This is interesting. A couple of months ago, I asked a biology professor on the internet this question. She gave me a very insightful and interesting answer. Read below:

Is it true that men cant settle with one woman while women can settle with one man?

She said:

No, it’s not true neither is it entirely false. The reality is a bit more complex than that. Our species is, depending upon how you choose to look at it, either serially monogamous or weakly polygamous. What I mean by that is this; in our ancestral environment (the EEA–Environment of Evolutionary Adaptation) the most stable strategy was mixed. If you were male, the most stable strategy would be to have offspring by as many women as you possibly could. If you were female your best strategy was to have offspring by the *best* man you could find. For women that may or may not be the man you were pair-bonded with. If you were a low-status female chances are you were only going to get a low-status male so if you could ever find a way to get pregnant by a high-status male, your offspring would be better off for it since it was the best of all worlds.

There are some very clear, if counter-intuitive reasons.

The reason why there are different strategies that are stable for men and for women only requires a bit of thought. For a male, parental investment *can be* as low as simply donating sperm while a woman has, at minimum, 9 months of pregnancy and then the next dozen or so years of near *total* offspring dependence. What that means is that for a male’s genes, quantity is going to have an edge over quality but for a female quality is *definitely* going to have the edge. A man can, if he chooses, impregnate a woman every day of the year and at the end of a little more than a year have 365 offspring while no matter *how* much sex a woman has, she’s going to produce no more than 1 child a year on average. (For the purposes of this discussion I’m going to ignore twins and other multiple births because they have no real selective effect.)

To understand just how powerful this is, do this little mathematical thought experiment. Take two men, we’ll call them Mr.P and Mr.M. They live in the same group and we’re going to hold the environment constant (because for this purpose it’s simply easier). The name of the game is leaving around descendants (i.e. copies of your genes). Those who leave around the most descendants wins. For sake of simplicity we’ll say that the reproductive cycle is 20 years.

In the first generation Mr.M has four children by Ms.M with 50/50 gender balance. In that *same* generation, Mr. P has two wives, Ms.P1 and Ms.P2. He has four children by *each* woman with 50/50 gender balance. The two sons of M (call them MS1 and MS2) each marry women and have four children each as do M’s two daughters (MD1 and MD2). At the end of the second generation the descendants of M are numbered 16. P’s *four* sons each marry two women and his four daughters each marry one man. At the end of the second generation the descendants of P are numbered 32 by the sons and 16 by the daughters for a total of 48. (4X4(2) + 4X4) Already P’s descendants outnumber M’s descendants 3 to 1. If P is genetically predisposed to polygamy and M isn’t it doesn’t take long before P’s genes will swamp M’s in the population.

However, there’s a price to be paid. It doesn’t take a great deal of genius to see that half of generation two will be male (24 grandsons of P) and if each of these grandsons marries 2 women and produces 8 children then the number of P’s great grandchildren will be almost 300. Of which half will ALSO be male. There’s going
to be fierce competition for females which is going to drive male body size up unless something is done to mitigate the effects.

One interesting effect of monogamy is that it drives *down* male body size. Keep in mind that poor M’s sons are trying to keep up with P’s sons and they can’t. If P’s sons then start competing for women, it will pay for them to be larger. One can see this in ‘winner-take-most’ harem species where males can be gigantic in comparison to females. It’s not because females like gigantic males it’s because the males have to fight to get a harem and then keep it! The less polygamous a species is, the less difference there is between males and females.

The fact that human males are *slightly* larger, on average, than females means we have some selective pressure toward male size–meaning we’re somewhat polygamous–but the lack of a huge difference means that we’re not fully polygamous.

It seems that eveything can be explained in terms of genes these days!