This is interesting and makes sense to me:
What the authors of Sex At Dawn believe—what they prove—is that we are a naturally non-monogamous species, despite what we've been told for millennia by preachers and for centuries by scientists, and that is why so many people have such a hard time being and remaining monogamous. I'm not saying that everyone everywhere has to be non-monogamous; the authors of Sex At Dawn don't make that argument either. (Lots of monogamists, however, run around insisting that everyone everywhere should be monogamous—and the monogamists get a pass because, hey, they mean so well and wouldn't it be nice if everyone were?)
The point is that people—particularly those who value monogamy—need to understand why being monogamous is so much harder than they've been lead to believe it will be. In some cases this understanding may help people find the courage to seek out non-monogamous relationships and/or arrangements and/or allowances that make them—gasp!—happier and make their relationships more stable, not less, as a routine infidelity won't doom their marriage/domesticpartnership/commitment/slavecontract/whatever. But understanding that monogamy is a struggle for most people, and being able to be honest with our partners about it, may actually help some people remain monogamous.