Well, here's an excerpt from Reason's Voter Guide:
Proposition 8: "…would result in California once again extending 'separate-but-equal' opportunities to its gay residents. If the rights are the same, the law should not distinguish between types of 'family relationships' or call them 'marriage' for one group of people and 'domestic partnerships' for another. A gay couple's decision to marry does not infringe upon a heterosexual couple's right to marry; so gay couples should be allowed the same opportunities and freedoms as heterosexual couples."
So, by process of elimination, I guess you know that it's a "fairly well-known libertarian think-tank" other than Reason that is still making excuses for Prop 8.
Thank you, Reason, for knowing that there's nothing libertarian about amending the constitution to define marriage via government fiat.