From a legal perspective I agree with the opinion (disclaimer: I didn't read it yet, based on my current understanding). Conservatives should see this as a victory for Federalism. If a state says that two people are married, where does the federal gov't come off saying they're not? Yet again, (ala Bush v. Gore) the Supreme Court justices all swap sides in order to stay in line with their policy objectives.
Had they found a constitutional right for gays to marry, that would be a different story, buy they haven't done that (yet).