Interesting.
The authors argue, and prove, that human sexuality has more in common with chimps and bonobos, especially bonobos, than with other apes and monkeys. Humans had a common ancestor with chimps and bonobos about 5 million years ago.
"Our DNA differs from that of chimps and bonobos by roughly 1.6 percent, making us closer to them than a dog is to a fox [...]"
Christopher and Cacilda show that in hunter gatherer societies, in which humans lived for much longer than in agricultural societies, the roles of men and women and the sexual norms were quite different from what is a norm today. Sharing was the norm, paternity was uncertain, possessions were few, conflict was rare, and food was free and not hard to find.
A few interesting societies are described. For example the Mosuo of southwest China, where people do not practise marriage, but instead have a system called "sese" - walking - where sexual partners are friends, not husbands and wives.
"The Mosuo are a matrilineal, agricultural people, passing property and family name from mother to daughter(s), so the household revolves around the women. When a girl reaches maturity at about thirteen or fourteen, she receives her own bedroom that opens both to the inner courtyard of the house and to the street through a private door. A Mosuo girl has complete autonomy as to who steps through this private door [...] The only strict rule is that her guest must be gone by sunrise. [...] There is no expectation of commitment, and any child she conceives is raised in her mother's house with the help of the girl's brothers and the rest of the community."
There are a few chapters about anatomy, which supports authors theory about humans prehistory being polyamorous rather than monogamic.
There is also a mention of one particularly practical research about the effect of contraceptives for women. Contraceptives which contain hormones affect the partners women choose. Women who are on the pill may choose a partner who they would not choose if they were not on the pill.
Finally, the book ends with a review of perils of monotony of current marriages and a mention of swinger's clubs of American WWII air force pilots and their wives.
