It will be interesting to read this brief after it is submitted to the court. Beyond the obvious problem that gay marriage itself is unpopular within the Republican party (only 1/3 in favor, but rising, according to the NY Times article this article pulls from), states rights/federalism arguments resonate with the American Right. They are arguably even more important to the conservative justices on the Supreme Court. Striking down Proposition 8 as unconstitutional is a significant blow to state power in a realm that is traditionally a state rather than a federal concern (marriage). Depending on the arguments they use, this pro-Big Government stance may be even more radical for these republicans than their pro-gay marriage stance.
That's an interesting angle. Is is a states' rights question or a civil rights question? And if both, which trumps? If it is ruled a civil rights issue, I don't think that states can pass laws that violate civil rights. But if it is a states' rights issue, doesn't that mean that other states to have the right to allow gay marriage if they choose?
This is the same predicament that existed for the interracial marriage debate (whether it was state or civil rights issue).