I wholeheartedly believe that monogamy goes against our human nature, because we ultimately crave for freedom. Some of us my repress those urges, and it is a likely occurrence due to the effect of socialization, but if you tap into them, any kind of relationship that limits you through exclusivity goes against those elemental urges. I think Lana's intro to Ride perfectly describes these urges.
EDIT: not sure of "exclusivity" is a word

exclusiveness (?)