If you reframe monogamy as a framework for romantic and sexual exclusion (inspired by the way I've frequently seen 'loyalty' and 'fidelity' used as synonyms for exclusivity) in service of one person's insecurity, it seems a lot less acceptable. We consider exclusion by a social group unacceptable in many contexts: racial segregation, gender discrimination, and religious intolerance are all forms of keeping the 'enemy at the gates' for fear of what intermingling would create. Most in modern societies mock, if not vehemently reject, the notion that excluding black people from white institutions is justified in the name of protecting white identity, or that keeping women out of male social organizations or the military somehow empowers men to be men, but the same people overwhelmingly embrace monogamy, a practice that says one romantic relationship is threatened by letting in even the possibility of other romantic (or even meaningful emotional) involvements. Why should we assume that to be true? Do we accept the idea that one friendship is threatened by the existence of other friendships? The demand for exclusivity is one that assumes the monogamous relationship is an emotional and sexual fortress or prison - depending on whether one is trying to 'keep in' (control one's partner) or 'keep out' (police those showing interest in one's partner) - which must be guarded lest something terrible happen. Monogamy maintains a fundamentally defensive posture around the boundlessness of love and the possibility inherent in curiosity and desire, believing the worst about human nature instead of trusting those who love us to continue loving us. The problem is that the insecure can never be loved enough, since insecurity has nothing to do with how much others love us, but with how much we love ourselves.