Lots of good stuff here. Monogamy is fine, if it works for you, but for a lot of people it’s considered the only real option, due to ancient societal rules and mores (which comes largely out of patriarchal control). As Shannon pointed out and I’ve said before, not everyone does the work necessary to go about healthy, ethical non-monogamy, but in theory and often in practice, it encourages better communication, better understanding of your own needs and triggers, and less co-dependency in relationships.
My partner James and I don’t do non-monogamy in a common way; we only see other people as a unit, but that has helped us making it successful for us, although it was still a lot of work and dismantling of old paradigms and programming to get to this place.
Dan Savage has a lot of good things to say on this issue, particular as far as ocassional slips or strays from strict sexual fidelity and how that shouldn’t be the basis for an otherwise good relationship being thrown away. He of course, advocates for doing this openly and honestly and not behind someone’s back, but I think it’s a very valid observation.