When someone today says they're married, do we ask if they said "I do" in a church or a courthouse? Nope. If they say they're married, they're married. Except when a gay couple says it, then the fundies say, "Oh no you're not!" Unless it was in Vermont ... or California, for a while ... or ... is it Idaho now? Can't remember.
But if we "gave" the word to religion, then people could say they're married, and no one can tell them they're not. And people who have a civil union will also say they're married. And since there's no legal definition of the term, no one can say they're not. Similar to "lite mayonnaise". There's no legal definition of "lite" so anyone can use it.