Good food for thought, Mono. I have seen a lot of relationships fail, in my own life and in my parents' (they've both been married and divorced three times apiece) but I think it was more due to incompatibility and inability to compromise than anything. It's hard to say for sure. Although maybe monogamy has something to do with marriages breaking up, because being faithful to only one person is so hard. I know for me, I've always wanted to be in a poly sort of situation, ever since I knew it was possible! As a kid, I thought relationships HAD to be monogamous. Finding out there are other options was an eye opening experience. For me polyamory is appealing chiefly because I'm attracted to men and women pretty equally, and when I'm with a guy I crave sex with girls and vice versa, but also because I think if you truly love someone, you should be able to allow them to be as happy as possible, even if that happiness is found with someone else. Sharing is just, well, less stingy. Heh.
I look at it this way... I have many friends. I love all my friends dearly, and I love different things about them. Some I'm closer to than others, but they're all important to me. I think we, as people, understand that we can have more than one friend, because they all share different interests that we have, and it's fun to spend time with them all for different reasons. One of them might share your love of Jazz music, one might love to visit museums, one is fun to go on vacation with, etc. We don't get hurt feelings if we call a friend and she's busy shopping with someone else, or if your friend wants to go see some geeky movie you don't like with another friend. You might even recommend it, "Oh, that movie seems completely dorky to me, why don't you just go with [Person X] instead? You guys would probably have more fun without me." And there's no hard feelings. Why is it different just because sex and romantic love is involved? I don't understand that aspect of humanity. I should be able to say, "You can share _________ with this person that you can't find with me. Go be happy."
I'd be lying if I said ALL my needs would be forever met by being with my SO, or with any one other person. The gender thing is a big one. I've had a lot more experience with men than women, and I feel I have some wild oats to sow, if you will. I don't want to break up with my SO just to experience these things, and I don't want to go through life gritting my teeth, fantasizing about girls when I masturbate or during sex, all the while lying to myself saying that's good enough. I don't feel it is. At times I have felt selfish for feeling this way, but now I think it's just part of human nature to be attracted to many people, even at the same time. It's part of my nature, anyhow.
I don't want to cheat, either, because lying is deplorable to me. So for me, it's about being honest and true to myself and respecting my partner(s) enough to be honest with them.
I know it would be selfish of me to expect my SO to be faithful while I sleep around, so I've got to be a big girl, as GroundedSpirit pointed out, and allow him the same freedoms he'd allow me. Plus I do love him and want him to be as fulfilled as possible. I hate playing video games, for instance. If he met a girl he could play video games with and sleep with and all that good stuff, why should that make me jealous? I know rationally it shouldn't, but it does. Because I am a jealous creature. And I hate it. Haha.