Did any of you watch my video? I linked to Dan Savage. He's been in a succesful open relationship for 15+ years with his life partner. He has some pretty convincing arguments against monogamy. Namely that monogamy is HARD and we're treated like shit when it fails, if you're with the person you want to spend the rest of your life with, and you give in to your natural urges, you are going to lose that person from your life. Which is ridiculous, just because you have sex with someone doesn't mean you have feelings for them, it simply means you found them attractive.
He says a lot of this much better than I can, youtube him.