Bad. Like most other people here are saying, we don't own our s/o's. I'd go further than most here and say that monogamy (serial or otherwise) is a form of possessiveness.

_ says, "If you don't really care what the other person does, are you really in a relationship?", but I'd say that caring what the other person does and trying to control what they do are two different things. Also, whether you're "really in a relationship" depends on how you define a relationship, and I think relationships are badly in need of a redefinition.