Ken Mehlman, former chairman of the GOP, is trying to sway independent-minded Republicans in New Hampshire to support gay marriage:

I will be in New Hampshire this week — to urge legislative members of my party to reject House Bill 437. It’s time to stand up for individual freedom and liberty, to live by the Golden Rule and to oppose any effort to diminish or strip away individual rights, and to return to the real business of building business, keeping taxes down and growing our economy. “Live Free or Die” should be more than just a slogan. -- The Union-Leader

NOM co-founder Maggie Gallagher has just published a research brief on this very topic: "Why Libertarians Should Oppose Same-Sex Marriage".

Ruth Institute President Jennifer Roback Morse has also published on the subject: "The Libertarian Case for Man Woman Marriage".