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.
There would be very quickly, or we would be right back where we are now.
Because either the two are legally defined to be entirely separate and that distinction is somehow forced on the public. Or the religious would be right back trying to outlaw civil unions for certain people, because the marriage word is only allowed to be used in certain ways.
To separate the two, you have to change the language. And doing that by government fiat is very hard.
Jay