And in doing so the government courts become the enforcers of the marriage contract. Do the people have a right to decide which contracts their government courts will enforce? I say that they do; clearly you say that they do not.
The people have a right to decide that a contract to violate the rights of a third party (e.g. a "hit" contract) is unenforceable.
The people do not have a right to decide that certain individuals have a "special right" to enter into contracts that are unavailable to other people (e.g. a zoning prohibition against home purchases by a disfavored ethnic group).
Simple, really.