I tend to agree with Stephan Kinsella. It's very hard to see how gay marriage will undermine heterosexual marriage in any practical way.
However, why should any libertarian support gay marriage in the first place? That is, how does supporting gay marriage do anything to lessen the scope and power of the State?
It seems to me that, in fact, gay marriage actually increases the power of the State, both by expanding the constituency for State services (in this case, government issued marriage licenses) and -- this is the most serious issue -- by using State power to enforce as binding upon society (not just the individualis involved) a contract that many of us think is absurd.
The result of gay marraige will be that the State will force businesses, individuals and institutions to treat gay marriages in exactly the same way as heterosexual ones. How is that libertarian? Right now, it's not at all a violation of libertarian ethics that gay marriage is not recognized; it simply means the State does not enforce this kind of contract.
There is no good reason that I can see for libertarians to support gay marriage, and those who do support are betraying whatever anti-Statist princples they might claim to have. They're sexual revolutionaries, not anti-Statists. That's how I see it, anyway.
