Lev Lafayette wrote:
The definition of marriage is not a static thing, and it is not owned by any religious authority. The age when marriage was considered acceptable has changed. Race restrictions once existed. As did religious restrictions as well, for that matter. So therefore the definition is mutuable.
In a modern society we're pretty much settled on the opinion that as long as it's between consenting adults, it's none of our business in a legal sense.
I remember asking once: "why does anyone even care? Unless you're some kinda wacky theist, it's totally irrelevant." And they told me that a bunch of old legislation is still running around, so e.g. in some places you cannot adopt a child unless you're officially Married™, and a bunch of tax stuff works differently. So AFAICT the quick-and-dirty fix is to pick a new word, like "froobznargl", define it secularly and without prejudice, then do a regexp replacement across all the legislation to switch to that term. That way, the credulous pre-Enlightenment types can continue to define "marriage" however they want, and it won't affect anyone outside their cults. Since the state is (at least nominally) secular, that won't be contentious AT ALL.