You have isolated the real problem, which is that fact that states are required to recognize marriages.
There is a lot of work to be done.
Marriages have always been “recognized” as legally binding, which means “by the state”. That way inheritance, protection of children, and so on are legal.
The ridiculous idea that marriages should just be private “contracts” with no public or legal ramification is a standard libertarian pie in the sky utopian “if human nature was all different, everything would be all different”.