This is an argument for marriages as contracts, but no more. I don't read an argument in what you say in favour of a government involvement in defining what a marriage is. Contract law should be sufficient.
I agree. Good point. There is a strong argument to be made for defining a prenup for all marriages, just as a contract would have a dissolution agreement. That way, the negotiation takes place when the people are on good terms and not trying to screw each other over out of spite.