My (monogamous) mate brought home a copy of Sex at Dawn, by Christopher Ryan and Cacilda Jetha. The book has generated a lot of excitement, probably (I haven't read it yet) because it argues that humans, like other monkeys, are not monogamous.
This is an ongoing argument, with many people pointing out that bonobos, the monkeys that are genetically closest to us, are enthusiastically polysexual.
So I'll take this time to point out, once again, two things:
First: monkeys, schmonkeys. Throughout the animal kingdom, there are species that are similar in many ways but different in their reproductive strategies, cf. the (monogamous) prairie vole and it cousin, the (nonmonogamous) montane vole. This crucial difference seems to lie in the distribution of oxytocin and dopamine receptors in the part of the brain that handles social relationships. When both these receptors are present, it seems to reinforce the preference for sex with a particular individual.
Second: Even monogamous species are not completely sexually monogamous. Even the adorable little prairie vole is not averse to a bit of copulation outside the pair-bond. So, probably humans are designed to have one primary mate, whether or not that includes lifelong sexual exclusivity.
Why I bring this up (again) today: A study by Charles Snowdon of the University of Wisconsin found that mated cotton-topped tamarins with mutual high levels of oxytocin were more romantic.
Okay, that's gratuitous anthromorphosizing. What he found, according to the university's press office, was that males in a high-oxytocin pair got more sex, while the females got more cuddling, making them both happier:
In the current study, the partners seem to know what the other partner needed. "Males in a high-oxytocin relationship were more likely to initiate cuddling, and females were more likely to initiate sex," Snowdon says. "These males were initiating the behavior that the female needed for high oxytocin, and the females with high oxytocin were initiating the behavior that male partner needed for high oxytocin."
Snowdon says. "Here we have a nonhuman primate model that has to solve the same problems that we do: to stay together and maintain a monogamous relationship, to rear children, and oxytocin may be a mechanism they use to maintain the relationship. Therapeutically, I'd suggest this would have relevance to human couples."
Theoretically, at least ...