I find the book to stray too much into unsubstantiated generalizations from a very limited source material. As an anthropologist, these kinds of books drive me crazy.
As a species (biologically) and as social/cultural beings, we are evolving. There is no one "nature" that we can point to. I'm not disregarding biology at all here, just cautioning that there is no human nature/essence that can be disentangled from culture imo. All things must be historicized.
I do like that the author is trying to show empirically that monogamy is not natural. Fine, that's a substantiated argument and a powerful one at this present moment. But, how people choose to live and love is fully entwined with cultural norms. Desire is hardly derived from a rational response to stimuli -- we can see that all around us.
I like to argue that we don't even have to go back so far, we just have to look at the diversity of intimate, family, child-rearing arrangements that exist at the present to see that there is no one type that is "normal."
"Love takes off masks that we fear we cannot live without and know we cannot live within."James Baldwin