Many libertarians are supportive extending marriage rights to same-sex couples, but not all of us are. Some libertarians see the legalization of same-sex marriage as detrimental to the quest for liberty because extending the institution of state marriage to more couples enfranchises them, making them less likely to oppose state intervention in marriage and to see the state as an opponent.
The libertarian position is almost universally that marriage is not the business of the state. It is the business of couples, their families, and, where applicable, their churches - but not of the government. If the state gets involved at all it should be through providing marriage contract enforcement.
But the world isn't binary - there are many different positions that can be taken by the state in marriage - some are better and some are worst. Anyone who expects government to abide by the rule of law ought to see same-sex marriage as an improvement, even though it does not represent the best possible outcome.
I had the privilege of discussing this with Jeffrey Tucker this summer and he raised an interesting question - if the problem with same-sex marriage is that it enfranchises more people and gives them a vested interest in the state, what makes it different than, say, legalizing (and taxing) drugs? Ending the war on drugs would certainly make a lot of people less likely to oppose the state.
Ending the war on drugs would let innocent people out of cages. It would reduce infringements on privacy and property by the government. It would drastically curb violence and crime. And it would, very likely, stabilize the government in a more restrained role. This could make progression towards the elimination and/or drastic reduction of the state more difficult. If the government taxes drugs (which it almost certainly will as they become legal) then it is unlikely that it will ever stop. We can expect rent-seeking and other outcomes of government involvement. Should the alternative, in the interest of achieving a stateless or minarchist world, be to carry on the war on drugs to keep skepticism toward and opposition to the state high? I consulted a friend who opposes same-sex marriage and he conceded that it might be the case.
Libertarians are justifiably skeptical of claims that anyone ought to give up their well-being, money, labour, or liberty for the "good of society." How can opposition to same-sex marriage or (in the more extreme case) to ending the drug war - that is, requiring that some continue to suffer so that all can be better off in the long run - be substantively different than the many demands and legal obligations that people put on one another through the state? What makes libertarians better judges of who ought to suffer for the greater good than the state?
Opposing equal treatment under the law treats people as means, not as ends. Refusing to allow suffering might not be the fastest way to a libertarian society, but it ought to be the position of any individual-centric ideology.