The state doesn't "sanction" marriage so much as simply recognize and record it. Otherwise, there would be competence testing before marriage, license renewal, and specific requirements for a marriage.
From a legal and cultural standpoint, putting “marriage” under the umbrella of contract law would do far more to protect it than meddling with a fundamentally flawed institution.
On the contrary, it would completely destroy it.
Marriage is NOT a "contract," although it usually includes one. It is fundamentally much more than that.
I know. That's why I put "marriage" in quotes in my prior post. The government had no business getting involved in marriage in the first place. I'm surprised churches haven't been more open about recognizing this. The government's only involvement (to the extent there even is one) is in the enforcement of contracts that may or may not have anything to do with a sacramental marriage.
The problem is that if marriage is something fundamentally more important than a contract, it nonetheless lacks the practical benefits that come with contract law.
Consider "no-fault divorce", which allows someone to petition for divorce without proof of wrongdoing by the other spouse, essentially nullifying the prior marriage contract...even if the spouse wanted to stay married.
That sort of provision, applied to any other area of life, would essentially make contracts worthless, because you wouldn't be able to trust other people to hold up their end of the agreement, much less trust that the state would be a neutral arbiter in the case of any contract disputes.
So why, then, should such an absurdity be tolerated for an institution that's so much more important in principle?