Andrew – I’m sure you’re right that many libertarians would and do make that case. Of course, it’s a bit of a funny argument when you think about it. Libertarians believe — most often correctly — that no one group should get special treatment from the state. But rarely do they make the argument that if one group is already getting special treatment — benefits, tax breaks etc — we should expand that treatment as broadly as possible. I’ve never heard a libertarian say that since Social Security is here to stay we shouldn’t just give money to old people, we should give it to young people too. Libertarians don’t say that since affirmative action for blacks is wrong but nigh-upon permanent we should therefore also give Latinos and Asians special treatment too. They don’t say that since we give subsidies to agriculture and steel we should also give subsidies to everyone else. Etc.
In other words, if benefits for heterosexual marriage are wrong, why do libertarians favor extending those benefits to homosexuals?