[dragonwriter]

> Because many Christians to varying degrees don't see marriage as a legal construct (whether exclusively, primarily, or at all, depending on the person), but as a religious one.

Whatever they “see”, it is, in fact, a legal institution, has been such as long as it has existed at all, and is a legal institution that in most modern liberal democracies (and many other regimes) has been entirely separate, and regulated differently, from any religious institution of the same name for quite some time.

There's lots of institutions like that, because religion and civil law weren't historically separate domains, and many institutions (minority/adulthood, for one pair) exist which formerly were shared between the two and continue to frequently exist in both domains.

That doesn't give religions the right to impose their rules on the public institutions, or to demand the public institution change its name.