I've been in several monogamous relationships, and a couple of open ones... I don't really have a preference, per se, but I do like the idea of sharing my life with one (ahem... specific) person. Sex is kind of different in my view, but I haven't had sex in about two years, so I might be a little skewed. Like others, I think open communication and like-minds are more important than anything else. If that (ahem... specific) person wanted to have sex with, say, a dude because she was missing it, I'd be jealous, for sure, but it wouldn't be a deal breaker by any means. Just no lying, and I'm good.
Nolite te bastardes carborundorum.