Almost no animals, particularly mammals are monogamous — only about 3–5%. Why would humans be? Cooperative breeding and child-rearing makes a lot more sense from an evolutionary standpoint because you’d have a better chance of the best sperm fertilizing the available eggs, and more children would live to in turn pass on their genetics.
As you are probably aware, there are few things in the social sciences (or most sciences) that are universally agreed upon, but more and more, it’s becoming clear that although humans may choose monogamy, it’s not the way that they are biologically wired. Believing that to be the case comes out of Victorian belief systems that Darwin took on, that were never properly challenged. Hoping to write something about that at some point in the near future.
From the shape of the human penis to the continuing practice of partible paternity to the thousands of years when female non-monogamy was a part of religious practice, a theory about human monogamy seems a lot more like a patriarchal pipe dream than anything substantive. There’s simply way, way too much evidence against it.