This week I am working through ideas from, or inspired by, Allan Carlson's Conjugal America. One rich idea is that sexual dimorphism – biologically based difference between male and female – is greatest "when sexual coupling is random or where one male accumulates numerous females" as with other primates. On the other hand, "dimorphism is least when male and female pair off in monogamous bonds."
Carlson cites a study by a Kent State research team led by Phillip Reno. They found evidence that our pre-human ancestors were both pair-bonded and more similar in size and shape than other, non-pairing primates are.
I think that marriage makes for more complementarity and true equality at the social level, too. Among human beings today, subcultures in which monogamy is rare also show the greatest animosity between men and women, and the greatest hierarchy when they do marry. Old marrieds, on the other hand, famously become more of a seamless team – not the same, but working together, equally yoked as one.