@TranscendHumanit,
This whole thread went a different way than I would have expected from this forum. I guess I'll go with the Lockean approach and say that the only things you have are the things you can control (in this case, the girlfriend or boyfriend is only committed to the other person for so long as that person meets all their needs and controls their affection). If they aren't meeting these needs, or you can meet them better, than the person in question is fair game.
Don't get bogged down in ethics, just like in a free market, if you are better for someone than their current match, then it is logical that you should be with that person.
I've never had a problem with guys trying to steal one of my girlfriends, if they can manage it, then obviously something isn't going right in my relationship.