The Strange Credibility of Polyamory essay by inki is good reading. Long but worth digesting the whole thing, including the comments. I especially like this insight (though it is bound to be controversial):
Monogamy, at its core, is about sexual fidelity, or rather, sexual fidelity is the one thing you need to be monogamous. Everything else is optional, like marriage, or living together, or not living with other people, or who you share secrets with, and so on. We can also see this in the contrapositive: having more than one sexual partner is by definition nonmonogamous. In other words, monogamy is actually "all about the sex", or rather who you have sex with is monogamy's first and most crucial requirement.