The freedom for consenting adults to gather in marriage in whatever form they choose. Might be 2 guys. Might be a girl and 2 guys or a group or what have you. As long as they are consenting and of legal age, (as required under most aspects of contract law) then let anything go. Marriage has taken many forms over humanities history. No reason it shouldn't evolve further. But this is getting off topic, which is what you seem to want.
As I asked in post 36, what gives us the moral authority that requires the union to be consenting?
Most, if not all of those civilizations with liberal "marriage" norms are history. I think one could easily correlate the downfall of the American culture with its liberalization of sexual moreys.
As far as taking this thread off topic, I disagree. It is typical progressive tactics to focus in incremental, progressive, changes to get to some desired state without considering where the road might end.