And they ought to.
So, I’m going to let you in on a little not-so-secret: childbirth is horrendously painful. Don’t believe those bastards with their natural birth hoo-ha. There is nothing magical or wonderful about childbirth. You bleed copiously, you can barely walk for the next two weeks, and before modern medicine, you stood a good chance of dying in the process.
The US’s homicide rate is 4.7 deaths per 100,000 people per year. The UK’s is 1. Japan’s is 0.3. (Source.)
The “developed nations” have a maternal mortality rate of 16 women per 100,000 births. (Source.) (Note that one woman may give birth to more than one child in her lifetime.)
The “developing nations” have a maternal mortality rate of 230 per 100,000 births (and a much higher number of children per woman,) and some countries have maternal mortality rates around 1,000 per 100,000 births. If the average woman in these countries has 5-7 children, that works out to 5-7% of women dying in childbirth. (Though not every year.)
In other words, in the state of nature, childbirth is kind of like being stuck in the middle of WWII.
Let’s consider the male side of things. Given enough available females, a male could, hypothetically, sire one child every night for 30 years, resulting in 10,957 children, plus or minus a couple depending on the leap years. We’ll call 11,000 our “hypothetical male maximum.”
The man stands approximately zero chance of dying in the process of siring children, does not endure pregnancy or childbirth, and even if he depends on someone else to do all of the childcare for him, he still has a reasonable chance at his offspring making it to maturity. (After all, Genghis Khan didn’t get to be the most evolutionarily successful man in recent history by raising all of his bastards.)
In other words, for men, there is basically zero cost to impregnating a woman (other than, obviously, finding one.) Even under harsh economic/environmental conditions, every additional woman a man mates with is an additional chance at offspring. The only limiting factor is how many women he can convince to mate with him.
A woman, by contrast, can produce at max only about 20 children (9 months gestation + 6 months nursing / 25 years fertility). (State of nature does not have baby formula.) She needs a maximum of 20 mates, and can make do with <1 if she has to share.
Where the male number of potential offspring is practically limitless, by human standards, the female is decidedly limited. Which means that women must be picky. A male who has sex once with someone kind of meh is not seriously limiting his ability to have tons of awesome children. A woman who has sex with a guy who’s kind of meh is potentially wasting one of her very limited chances of having children on a loser.
Note: I am not claiming that 11,000 is a realistic number. Obviously even Genghis Khan himself probably didn’t have 11,000 children. The point is that male and female reproduction are vastly different, creating very different incentives.
The average male has approximately zero to lose and all the offspring to gain from a random fling, and thus is mentally ready to consider the majority of women as potential mates. If some of those women happen to not be particularly attractive, well, it’s no big loss.
Women, by contrast, endure high-risk pregnancies and births, for a very limited set of children. They therefore cannot risk having children with inferior males.
So it is in the male’s interest to mate with everyone in sight, but in a woman’s interests to eliminate the vast, vast majority of potential mates, winnowing her selection down to the best 1-4.
Which means that all other potential suitors, whether they’re 14 men in a small hunter-gatherer band or 4 million men in Tokyo, are sexually useless.
In fact, they’re less than useless: they’re a threat. Because it takes only one act of violence for one of those men to get another potential offspring, and for a woman to lose one of hers.
The difference between optimal male and optimal female strategies leads to conflict–not just between men and women, but also between men and men. If one man mates with 550 to 11,000 women, (11,000/20 children per woman = 550), that implies that 549 to 10,999 men are not mating with those women. And those men are going to be pretty pissed.
Monogamy is one of the more elegant solutions to this issue. The relative guarantee of one mate per person reduces conflict and increases parental investment in the children. But monogamy requires fidelity–reducing female interest in the sexual desires of the majority of men even further.
Disgust in the face of unnecessary male sexual attention helps keep women loyal to their husbands and protects their long-term evolutionary interests, even if people express it in really annoying ways.