Multimarriage is different, as I described.
Letting The Gays get married doesn't change any existing law at all other than the daft bit that constrains the gender of people getting married. Everything else is the same - dying, next-of-kin-ness, automatic stuff that happens on death, etc etc etc.
All that law exists and is well-understood.
It simply doesn't exist for multimarriage.
Your "logical extension" is nothing of the sort, and is indeed akin to the "oh em gee, they'll be getting married to their pets and their motor vehicles, next!" stuff.