The book shakes the foundations of what's "Natural" in the human animal's sexuality. Core thesis:
Humans of neither gender are naturally monogamous, and expecting them to be is NUTS.
Core evidence: Look at the 2 other animals we are closest related to: Chimps + Bonobos. Also, look at human biology. Also, look at non-propertarian/pre-agricultural cultures.
1. Evidence from modern pre-ag societies:
Pre-property, marriage either doesn't exist, or doesn't assume sexual exclusivity.
Other folks writing on the topic (Pinker, for instance) discuss primitive folks (Yanomamo, for instance) who are at least 50% an agricultural society...which isn't representative of pre-agricultural societies. Summarizing a quote from the book: Pinker discussing forager societies is like hearing someone discussing African societies, and picking 7 examples: Vietnam, China, Malaysia, Japan, The Philippines, India, and Saudi Arabia. Saudi Arabia is arguably African, but barely...and the other 6 aren't at all. Repeating the core thesis: If we look at non-agricultural, non-food-storing, forager-type societies...sexual exclusivity is at best a rare outlier, and more likely, non-existent.
2. Evidence from common ancestors:
Unlike most other known animals/mammals, our two closest genetic relatives (Chimps + Bonobos) both are highly non-monogamous, and highly sexual. Whereas other apes/monkeys/mammals have only rare, fertile sex, and usually in a polygynous arrangement, our two closest genetic relatives have frequent, multi-partner, fertility-agnostic sex.
3. Evidence from biology:
If you look at human beings...it's fairly obvious that of ALL animals, humans are among the best-designed for sperm-based competition for reproduction. Basically...our enormous, mushroom shaped dicks and massive testes (or mine anyhow) make sense evolutionarily only in an environment of substantial sperm competition: multiple men mating with a single woman.
3b. Other, minor evidence from biology:
Female mating vocalization. Rare in animals...serves to make other nearby men aroused. Evolutionary purpose?
Female orgasmic patterns. Average duration to orgasm per woman vs. average duration to orgasm per man?
We roughly have pretty strong evidence that pre-agricultural pre-historic humans and pre-humans (10KBC back to 10MBC) were nothing that even resembles monogamous. Multi-male multi-female mating has been the norm in our ancestors for the last 10M years, with a minor blip in the last 10KY.
Aretae speaking now, no longer just summarizing:
The modern institution of marriage is a result of two factors:
B. Male Ownership.
With property, which exists for the first time (for real) in Agricultural Society, there is a question of how property passes from generation to generation.
With Male Ownership of property, men have a desire to pass property to their own genetic descendents. Therefore, given that they own property, they can force the women to do their will, and then work very hard to control the naturally promiscuous human female.
The fact that we have psychology hacks, and ownership fears, and that game-players can lie (stretch the truth) so as to get what they want doesn't make it natural, or even tolerable to human psychology of either gender.
Again, the Aretaevian prediction:
Marriage is dead within 30 years. As the current age of scarcity comes to a close, marriage has no way to survive the natural disinclination of both sexes towards monogamy.
Unfortunately, the historical replacement for marriage seems to have been clan-level poly-fidelity (+swinging with other-clans) , with communal child-rearing. And the child-rearing property-transferring purpose of marriage is NOT being served by any current arrangement, unless it's the 6 modern poly-fi clans.
May you live in interesting times.