I don’t think that is practical. There are literally tens of millions of government recognized contracts that are already in application that rely on the consistent legal definition of those terms.
There are also, at any moment, thousands of cases before the courts involving said contracts.
Just getting the government out of it would be a legal nightmare and a boom for lawyers redrafting millions of private contracts.
Marriage is a legal term and thus is under the jurisdiction of the government to define for consistent application.
Of course, there is also the Ex Post Facto issue regarding a recognized contract that is no longer recognized.