Marriage is, at its core, a social contract. Two adults pledge to be responsible for each other. If they want to add a spiritual or religious element to it, they have the freedom to do so.
If marriage were purely a faith-based institution, the federal and state governments would have no legal basis for enacting any laws relating to marriage. Adults would only be able to file taxes as individuals – at most their spouses would be dependents (a whole other can of worms). Spouses would be obliged to testify against each other (maybe that’s just a TV thing). There would be no divorce – good luck getting your records after the split.
On June 6, will it be legal for all consenting adults to marry each other, regardless of gender, or will there be enough religious fanatics in Washington to force the issue to a popular vote? We’ll just have to wait and see.