The idea is that "marriage" becomes a church thing, or a social thing.
Under this idea, you don't get or lose any legal rights by "marriage". You get or lose legal rights by civil partnership.
And as such, how Episcopalians* define "marriage" makes about as much difference as when two kindergartners decide to play wedding.
I actually see this as inevitable in the long run. "Marriage" has too much baggage that is utterly incompatible with how we do government. You can't have privacy and gender equality and then ban gay marriage without also banning marriage (as a legal status) in general. I don't think legal marriage is viable even with gay marriage, but it could be.
You have to remember, the judges that ruled that marriage can't be straight-only were not gay activists, or even particularly liberal. They were just following the law to its inevitable conclusion.
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* or any other church