More of a one-to-many rule. Only one side is expected to be fully monogamous.
It's long been socially acceptable for men to have mistresses or even multiple separate families, so long as they had the resources to take care of them all. And the social faux pas of merely sleeping with other women is very recent.
Biologically, there are differences.
Typically, societies with imbalanced relationship ratios, an in particular single males, tend to be more unstable. Should poly folks design their life around the consequences of disaffected young males? No, of course not. Nor should we artificially privilege monogamy to ensure social stability for obvious reasons of individualality and moral policing. We should study the phenomenon and remedy the male psyche to ensure social stability and discover, scientifically, the threshold at which we can expect it to be a problem.