No state shall ... pass any ... Law impairing the Obligation of Contracts ...
(U.S. Constitution, Article I, Section 2)
Which raises an interesting question. The recognition of gay marriage by a state changes the meaning of terms, such as "spouse," that are used in private contracts. Suppose an employment contract includes health insurance for the employee's spouse. Does that mean that, when gay marriage is legalized, same sex spouses are automatically covered by existing contracts? If so, the state would appear to have changed the terms of, which would seem to violate the contract clause.
In that particular case, one could argue that the change expands rather than impairs the obligation, but one can easily enough imagine the reverse situation, where the result of a legal change is that some people who had been considered spouses when the employment contract was signed ceased to be.
And if the answer is that changes in the meaning of the terms in a contract do not "impair the obligations," it is hard to imagine anything a state government might want to do along those lines that could not be disguised as a redefinition of terms.