Adam Phillips again. Here is what he says about monogamy that I find quite interesting:
Like a magnet that collects our virtues and vices, monogamy makes the larger abstractions real, as religion once did. Faith, hope, trust, morality; these are domestic matters now. Indeed, we contrast monogamy not with bigamy or polygamy but with infidelity, because it is our secular religion. God may be dead, but the faithful couple won't lie down. (Monogamy, p.10)
There are indeed people whose understanding of monogamy comes from their religion, but Phillips are not talking about them. What he suggests is that people, including (or particularly) those who do not have a religion, need monogamy the way they need religion.
This sounds to me an exaggeration. I do think, however, he is right that we contrast monogamy with infidelity. This raises a question: How is infidelity in polygamy different from infidelity in monogamy? In polygamy, when the husband (or the wife if it is polyandry) is unfaithful, he is unfaithful to more than one person. Is it worse than, or not as bad as, being unfaithful to just one person as in the case of infidelity in monogamy? Conversely, when one of the wives is unfaithful, can we say that she is unfaithful only to part of the husband because she shares him with his other wives? If we can say that, should we also say that her infidelity is not as bad as that in monogamy?