...That's a hard one. That's a really hard one. I believe in open-ended relationships, as long as all involved are aware what's going on and there's 100% communication and understanding about the nature of the relationship. I believe I could be in one. Sort of. I don't like to admit it, but against all my objective, technical definitions of morality...I would probably get jealous. Or maybe I'm just thinking of the person I'm with right now, who I'm starting to care for (possibly love) a whole lot, and I wouldn't care as much if it was just some random hookup and I was free to play the field too.
tl;dnr - Yeah, I believe an open relationship can be a perfectly moral and healthy thing, but depending on feelings you can't control, it may not be possible in some situations.