Is Monogamy Unnatural?

I'm in the middle of Sex at Dawn, the book that's caught the attention of a number of commentators, including Dan Savage and our own Andrew Sullivan.  I'm about halfway through the book, and so far, I'm disappointed to say that it reads like horsefeathers.  As someone who's wary of evolutionary biology stories which just happen to tell us that our dominant social structures are "natural", I should find the book interesting.  Unfortunately, it reads like an undergraduate thesis--cherry-picked evidence stretched far out of shape to support their theory.  The language is breathless rather than scientific, and they don't even attempt to paper over the enormous holes in their theory that people are naturally polyamorous.

For example, like a lot of evolutionary biology critiques, this one leans heavily on bonobos (at least so far).  Here's the thing:  humans aren't like bonobos. And do you know how I know that we are not like bonobos?  Because we're not like bonobos. There's no way observed human societies grew out of a species organized along the lines of a bonobo tribe.

Besides, as Jesse Bering points out, jealousy ('heartbreak") "throws a monster of a monkey wrench into the evolutionists' otherwise practical polyamory".  If we're evolved to be polyamorous, why do we also seem to be evolved to be extraordinarily possessive?  This seems like an evolutionary maladaptation.  And I find it hard to believe that this is just a cultural quirk, given that it does appear to be cross cultural, and it doesn't fade much over history the way that, say, attitudes about female dress have.

Lifetime monogamy may not be the evolved human template.  But I'm pretty sure that carefree polyamory isn't either.  And at some level, who cares?  Rape seems to be pretty "natural", but I'd still like to build social institutions that fight this "natural instinct".  The book might have been thought-provoking, but so far, in trying to prove too much, they end up proving nothing at all.  And the "I bet you didn't know about . . . bonobos!!!!" tone is incredibly off-putting.