We get some substantive and excellent comments at IGF, but Tom's essay in response to Steve's post on Rand Paul stands out. Tom's essential point is this: It is all well and good for a libertarian (or anyone else) to stand on principle. But once others have compromised your principle, what do you do?
Specifically in the case of same-sex marriage, Paul The Younger has said that he doesn't think the government should be in the marriage business at all. I'm not sure, myself, and can see the arguments on both sides. But he's acting on this principle only to protect same-sex couples from this governmental intrusion into liberty, while resting comfortably in the reality that opposite-sex couples are suffering daily with no end in sight. He has a similar position on the Civil Rights Act of 1964, which he doesn't think was good libertarian policy to begin with; but apparently he can live with it now. However, I'm sure he'd continue to protect the privilege of lesbians and gay men to be free of this overbearing encroachment on their rights.
The greatness of libertarianism, in my view, is that it has the proper respect for the constitution, which after all is a limitation on government. One of its principal limitations is that, when the government acts, it must do so equally with respect to all groups, and may not pick and choose among them without very good reason. Perhaps the constitution really does prohibit the government from entering into the marriage business. Or perhaps that's just a better understanding of government's proper role, irrespective of the constitution. But no court has ever ruled that this is an illegitimate function of our government, and I'm not expecting one to any time soon.
As long as that is true, then, a libertarian -- if he or she is truly principled -- must take one of two positions: either take a stand to challenge the entire misbegotten exercise of unauthorized and/or immoral authority for everyone, or else make sure that this power, as it is being exercised, is exercised without illegitimate discrimination among groups.
That constitutional libertarianism is how we should measure Rand Paul's brand. So far, his principles seem a bit more politically convenient than constitutionally defensible. Or principled.