Obviously all or at least most Libertarians agree that we ultimately are uninterested in legal marriage altogether. It should be just between the individuals being married, and if their religious then they're God or possibly Church. Mosque, Synagogue, Temple ect. You shouldn't need a certificate from the Government at all.
The question is as long as we do have legal marriage, should we not care about the what kinds are allowed? Some Libertarians seem to take that approach and focus only on the ultimate goal. And I feel they do this largely to try not to alienate social conservatives and Christians.
But I believe we need to firmly hold that as long as it is a legal institution there should be no restrictions on it. Gay Marriage, Polygamy, Polyandry, Polyamory, Group Marriages, ect. Anything between Consenting adults. I believe it's taking away the state's ability to restrict it that will cause legal marriage to crumble.
We're not gonna get our goal over night. Just as a full end to the drug war begins with legalization of Marijuana. So the war against government control of marriages has a beginning fight too.
I know some feel like expanding legal marriage is only expanding government power. I feel in this context legal marriage should be viewed just as a binding contract more them more then one indivdual, and and eligibility for that contract should not be limited.
As I said before a desire to not alienate "conservatives" is why some Libertarians don't want to say that. Problem is I feel the Ron Paul campaign of 2008 and 2012 won us all the converts we're gonna get out of the right for the time being. It's that the Left sees us as merely a variation of Conservatives that we need to counteract in order to grow our movement.
At any rate I've elsewhere on this Blog I've laid groundwork for how to convince Fundamentalist Christians to be ok with legal Gay Marriage, whether they morally approve of it or not.