With a small number of important exceptions to which I'll return in a moment, ever since Erie RR v. Tompkins, federal law, including federal constitutional law, has been indifferent to the source of state law. Erie held that the common law as announced by state high courts is state law to be applied by federal courts in diversity (and in what are now called supplemental jurisdiction) cases. Indeed, even prior to Erie, federal courts accepted as authoritative state court constructions of state statutes.
The exceptions fall into two principal categories: 1) Where state law questions are intertwined with federal questions, federal courts need not accept state court determinations of state law that defeat federal rights; and 2) Particular federal constitutional provisions in Article II and the Twelfth Amendment limit the deference to which state courts are otherwise entitled in interpreting state law. Three of the Justices in the majority in Bush v. Gore (Rehnquist, Scalia and Thomas) relied on point 2) in rejecting the Florida Supreme Court's interpretation of its election law as unreasonable.
Now comes Fred Thompson proposing a constitutional amendment that would draw the sharpest distinction ever between state judge-made law and state law embodied in official text. Thompson's proposal (described in a wire story here) would invalidate state court judicial decisions recognizing a right to same-sex marriage but would permit states to recognize same-sex marriage by legislation (and presumably by referendum or by a popularly ratified state constitutional amendment).
To be sure, the fact that federal law generally draws no distinction between state judge-made law and state legislation is not a reason why the federal Constitution cannot be made, via amendment, to draw the distinction with respect to same-sex marriage. But there are at least two reasons to question Thompson's proposed distinction (apart from the sound moral reasons to oppose amending the Constitution to endorse second-class treatment of gay and lesbian Americans): First, in many states judges are elected, and the state constitution is easily amended, so that if people worry about state court judges acting contrary to the wishes of state electorates with respect to same-sex marriage, there are ample means to respond, without amending the federal Constitution. Second, one can imagine nice questions arising in the event that a state court holds that state legislation or constitutional provisions in fact do provide for same-sex marriage. Would that satisfy Thompson's amendment? How clear would the state legislation or other text need to be before it would count as an enactment rather than a judicial interpretation?