I will guess that for 99% of Man's evolutionary past, that man, woman, and kids was not the typical way children were raised. Just looking at contemporary hunter-gatherer societies (what's left of them) shows one that. Many different kinds of arrangements have been used.the theory that society operated under for years is that marriage between and man and women was a good thing because it was typically the best ways to support and protect children.
when society is willing to go against such an obvious thing and pretend other bonds are equal... society has ceased attempting to be logical.
so how can you "discriminate against polygamy or community marriage? thereis no remaining rational basis for "discriminating" against polygamy... that I have read.