Mark Levin is correct that the Massachusetts gay marriage decision may well become a federal constitutional issue due to the Full Faith & Credit clause, but this does not make the controversy any less about federalism. Federalism is not about simply letting the states do what they want — indeed, it’s not about “states’ rights.” Rather, it is about the proper division of authority between the federal and state governments. Essential to the American notion of federalism is that states, while sovereign over most internal matters, should not be able to extraterritorialize their policy decisions on other states. Thus, preventing the Massachusetts Supreme Court’s decision from dictating policy outcomes on gay marriage in other states through the Full Faith & Credit clause is fully consistent with federalism — indeed, I believe it is compelled by a proper understanding of federalism. It has long been recognized that, in proper circumstances, states may assert valid public policy reasons for refusing to provide Full Faith and Credit to certian contracts or legal arrangements entered into in other states. I think Mark and I actually agree on the substance of this point. Where we may disagree is whether federalism principles also suggest that Massachusetts should be able to recognize whatever sorts of unions it wants to within Massachusetts, and Massachusetts alone.