A commenter in an earlier post mentioned this piece at TechCentralStation, by James Miller, arguing that libertarians can't appeal to a "hands-off" principle of neutrality to decide the gay marriage question, and so are forced to "take sides" in the culture war.
First, this is a bad argument. Just as failure to restrict an activity does not constitute endorsement of that activity, neither does equal access to government benefits imply endoresement of those who accept the benefit. On Miller's premises, the repeal of anti-miscegenation laws "endorsed" interracial marriage and, presumably, the Supreme Court wrongly decided Zelman v. Simmons-Harris, where it ruled that neutral vouchers guided by the "true private choice" of parents did not count as state endorsement of Catholicism just because parents might use them at parochial schools. I await Miller's announcement that he believes that vouchers offend the Establishment Clause.
Second, this is an issue only if you buy Miller's earlier argument that marriage shouldn't be an entirely private affair— that argument is actually significantly less compelling, and indeed, confused in enough delightfully different ways that I'm not even going to try to deal with it here.
Anyway, the same article was linked by the right honorable opposition over at the Corner, and Jonah Goldberg offers the distinct (and rather better) argument that libertarians don't normally accept the notion that a government benefit that's inappropriate in the first place should be extended evenhandedly. So, for instance, we don't want to expand affirmative action to include more groups, or pay Social Security benefits to everyone (not just the old).
First, I think we can certainly come up with a number of counterexamples. I mentioned vouchers above. We might prefer to have government exit the education business, but many of us regard it as a second best to at least let parents choose the destination of those tax dollars from among a wider range of options. And I'm pretty sure that there are very few libertarians who would defend legislation that excluded some disfavored ethnic group from receiving postal service, student loans, or welfare benefits, even assuming that this were politically feasible and would realize tax savings.
Note also that the examples Jonah picks don't work all that well. Affirmative actions is, more or less by definition, dependent on inequality. If you extend it to everyone, it's the same as extending it to nobody. Then there's Social Security, which at least in theory (if not in practice) is supposed to be "equally" available to all. Every old person was a young person at some point, after all, even if the converse doesn't hold. And here, the "second best" solution libertarians are inclined to accept for the time being (i.e. private accounts) would be a way of remedying the present inequality in the system—not between young and old per se, but between those with longer and shorter lifespans. The ability to bequeath or borrow against a personally owned account is, indeed, a way of distributing the (admittedly improper) Social Security benefit more equitably.