I'm with the McGuffin on this: Marriage existed long before Christianity, so if anyone gets to give up the word, it's the religionists. Or, heck, let's be generous and let them keep using the term, provided they use it with a qualifier: "Religious marriage", or something. As long as everybody knows that's not what actually counts for anything in the real world... But the word "marriage" in itself is part of language just like any other word, and there's no sensible reason to let the religionists co-opt even some small part of that.

(That's actually how it works, AFAIK, in traditionally deeply Catholic but legally wholly secular France: People "get married" twice. Once [usually first, AIUI] legally at the magistrate, and [then] optionally also in church, by a priest. But the latter is only "for show" -- for any deeply religious mothers and aunts who think that's "the real deal"; and, at a guess, in no small part to give the bride the grand show she's always been dreaming about.)