There's no evidence that suggests monogamous relationships are inherently better than any other kind of relationship.
Actually, one need only look at history to see that societies with stable relationships as the norm have done markedly better than those where chaotic familial relationships are common. Monogamy, as the simplest form of such a relationship, is the easiest to maintain in a society where the sexes are considered equal. Polygyny and polyandry with a defined head of household can work in societies with strictly defined gender roles, but that doesn't apply to modern Western society. While in theory equalitive polyandry can work, and for some people it does, in practice having more points of potential failure in the relationship makes it more difficult to maintain the stable familial relationships a society needs to function well.