Well, I'm not sure if he's a Republican, but he sure has a conservative pedigree. Quoteth David Boaz, via Reason:
In other words, this “liberal San Francisco judge” was recommended by Ed Meese, appointed by Ronald Reagan, and opposed by Alan Cranston, Nancy Pelosi, Edward Kennedy, and the leading gay activist groups. It’s a good thing for advocates of marriage equality that those forces were only able to block Walker twice.
He also notes that two other very prominent pro-gay marriage judges – the one involved in Iowa's surprise gay marriage legalization and the one in Boston who overturned DOMA – were also appointed by Republicans.