SSM and polygamy are not really related issues. You run into very complicated issues when you consider how children and property ownership would work in a many-person relationship. Marriage specifies shortcuts for these kinds of things, but they only work for two-person relationships. Polygamous relationships would basically require unique contracts given all circumstances, which is a pretty high burden. It's also not clear that polygamous marriages would have the same kind of societal benefits that two-person marriages have.

I think it's a point that can be convincingly argued, but the legalization of SSM does not imply a requirement for polygamous marriage's legalization. They're entirely different issues.