"Gay marriage" in fact represents a vast expansion of state power: In this instance, the state of New York is declaring that it has the competence to redefine a basic human institution in order to satisfy the demands of an interest group looking for the kind of social acceptance that putatively comes from legal recognition.
There is a curious rhetorical fact that has usually gone unremarked in these debates, but which is worth pointing out. That what the New York state legislature approved has to be described, not as marriage, but as "gay marriage" or "same-sex marriage" is itself a verbal indicator that what is being done here is counterintuitive. We all know, or thought we knew, what marriage is, and to add the qualifier "gay" or "same-sex" is a tacit admission by the proponents of the practice that it requires an appeal to authority to enforce what seems strange, odd, not right. The verbal tic of "gay marriage" or "same-sex" marriage is thus itself a rhetorical warning sign that what was done in Albany was an exercise in raw state power, the state's asserting that it can do X simply because it claims that it has the power to do so.
And that is an exercise of power that libertarians ought, in theory, to resist, not support.