[Jtsummers]

The fundamental change (ignoring issues with the religious) for same-sex marriage really was just the genders specified/described in most legislation and regulations related to marital rights, privileges, obligations.

For multi-party marriage one major issue will be based on this assumption: members can enter into and leave a marriage without dissolving the rest of the union. If 4 people are married and wish to add a fifth, and later one wants to leave, this should be feasible. How will the various assets be combined and removed from the other parties when people enter into and leave this plural marriage?

It's not as straightforward as the current two-person marriage. (Ha! It's not clean for two-party marriages either.) It'd be interesting to see, but it'll require a bit more structure to be set up, or at least more deliberation to ensure that it's not a complete clusterfuck the first time someone wants to divorce their two wives and husband (see also the issue with civil unions before same sex marriage was legalized and the issues faced when trying to dissolve those).