I kind of disagree... To me fidelity means being faithful to the commitment you've made not to have outside partners. It can happen is a mono relationship or a poly one. If there is commitment but no faithfulness, it's cheating. If the commitment isn't there in the first place, the relationship is open.
A relationship that is poly AND closed might sound weird but it's not that uncommon I would think.
Oh, and to me promiscuous means horny tease, but doesn't imply actual sex. But I guess I'm wrong about that? I always thought it was about the attitude, not the actual actions.