I used to think that the government should just take "marriage" off the books.

Marriage as a legal construct (not a sacrament, I couldn't give a rat's ass about sacrament for the most part), though, is important. It's given standing in society and law that basic contract law is not afforded.

I've watched too many people who have done their due diligence, created ironclad wills and trusts, and still lost their whole lives because some outsider estranged "family" member came in with better sleazier lawyers and got the contracts and agreements invalidated.