This is a very good point, but while we can't reasonably expect the government to impartially enforce all contracts presented to it, we also can't reasonably tolerate a government that won't recognize or enforce any contracts.

To take an excerpt from one of the linked essays

some couples have specifically contracted for the rights marriage traditionally gave them (but no longer does). In the California case Diosdado v. Diosdado, 97 Cal.App.4th 470, a husband and wife contracted that if the husband had an affair with another woman, he would pay the wife $50,000 on top of the divorce settlement, and vice versa. The husband did in fact have an affair, but the California court refused to honor the couple's agreement. The strong California public policy of no-fault divorce, the court said, prohibited courts from even enforcing the voluntary contracts of a mature adult couple

This is not a contract that goes beyond my intuitions of what it would be reasonable for the state to enforce. It seems we've come to a point where the state (or at least, some states) will refuse to enforce contracts for marriage with terms which it would readily enforce if they were business agreements. Certainly there are stronger economic reasons for making business contracts enforceable than marriage contracts, but I don't think it follows that marriage agreements should be less enforceable, and I have doubts that this is an optimal state of affairs.