Openness and Polyamory aren't the same thing. HAPPY seems to be in some sort of legit Polyamorous relationship. NNP is just in an open relationship, the terms of which he isn't happy with. Open isn't poly. Stop using them interchangeably. If partner is just having "novel experiences" (i.e. sex) with people other than the LW, and not actual relationships with those people, she's not "having polyamorous relationships." Sex doesn't always equal "relationship." If LW came to terms with that, maybe he wouldn't be so crabby about the arrangement. Or maybe he should just find someone who wants to be monogamous with him and leave her, so everyone could be happy.