Lestat's post makes some good points. I'll add though that aside from being unnatural, monogamy in our society is something we all generally want from our partners for a few reasons (feeling wanted, needed,etc and also for security and health concerns, especially these days), and want to give in return if we aren't hypocrites, or maybe some out of fear of social stigmas if they aren't. A deeper commitment to monogamy is made when the brain chemicals of love are thrown into the mix. In this state, cheating would seem absurd, and you only have that one person on your mind. I've often wondered what makes people fall in love; some to do with unique feel-good experiences that the brain connects to a certain person, which releases "love" chemicals. I'm no scientist, but mutual love is about the best thing life has to offer, Its truest form is tough to "find" though. Sex and love are also usually independent of each other, but are naturally best when combined.