I think you have at least one good point here, but I believe your critique contains several misconception. Let me deal with those. First, there is absolutely nothing New Age about this book. No cyrstals, no past life regression, no love signs, acupressure, reiki, homeopathy, or anything of the sort. Second, over the last 10,000 years (not 15), monogamy as we know it has not been the universal norm; polygamy has been at least as common (read your Bible!). And until the last couple of centuries, monogamy involved treating women as chattel. Finally, one thing is very clear, and that is that monogamy is NOT working like a charm for "the vast majority. Anything but. The rates of divorce and infidelity are eloquent testimony to that. In many Western countries, traditional monogamy has morphed into serial monogamy, which is really a form of polygamy, as the standard.
Now the term "monogamy" simply means one marriage, but we usually assume that it involves sexual fidelity. The authors, themselves married, seem to believe in the value of a life-long partnership. But you are right that the authors do not suggest any alternatives, other than suggesting that sexual infidelity alone should not end an otherwise successful marriage. The whole point of the book is that we evolved for 190,000 years as strongly promiscuous animals, and that while we can choose to behave differently, we're swimming against the stream, just as, for example, Roman Catholic priests do.