For me the issue is not whether or not it's institutionally legitimized but whether or not it's held to be actionably illegal.
There ought to be nothing illegal about it. As long as people aren't harming non-consenting (adult, human--I know for this sometimes idiotic crowd I have to specify) others, they should be allowed to live in what arrangement pleases them.
But that doesn't mean they should be allowed to benefit more so than everyone else by legitimizing polygamous marriage.
Will every spouse become a tax shelter, does each non working spouse get their own individual retirement account with those benefits, estate taxation, school funding through property taxes (the children of 50 wives living on one homestead). What of company insurance plans covering families? Etc.
So for me the issue isn't a matter of morality or legality but of practicality and equality.
Is that demonstrated or could exemptions be written in, with, say, a primary spouse as the only one of the other otherwise legal spouses who would receive those benefits? But that introduces unfairness into their family. It might be self defeated that an apple wants to be an orange. I tend to think that by definition, they place themselves outside that system. I don't see that as having anything at all to do with marriage equality for gay people.