(See: The Libertarian and Conservative Case for the Abolition of Marriage Laws, via Serge's blog.)
I'm being sarcastic, but on the other hand, the question of which sexual unions society will regard as legitimate and worthy of legal protection is pretty fundamental.
What Peter Reith (the author of the linked essay) argues for, intentionally or unintentionally, is theocracy, at least for those who want it. The Church will decide whether you get to break the marriage contract, adopt babies, inherit from a family member, or collect child support. Now, I may be in total agreement with this, but it violates the charter of every secular, democratic State out there, and for good reason, from the State's perspective. Reith's presumed point--the secular State should give up its monopoly over the courts--requires the State to sign its own death warrant.
I think libertarians like Reith want to have their cake and eat it too. They enjoy the prosperity afforded by the secular State's economies of scale but want it to carve out religious enclaves and everybody can just get along. But we all pay taxes, we all pay insurance premiums and we all have to deal with other people's bad outcomes. Monogamous heterosexual, polygamous and homosexual unions have consequences. Hence the citizens make policy choices among the various options. Substitute the State for whatever social structure you like: Amish plantation, Hebrew township, Somali clan, Muslim caliphate. In every one of those places, somebody is going to end up beyond the pale.
The only reason we are having this debate is because we are numerous, even antithetical cultures all under the same State. The debate will ultimately be resolved by the State as final arbiter because it has the most guns. It's pretty easy to predict that the State will extend legal validity to homosexual and other unions because it wants as many constituents as possible. And here is where Reith's argument gets slippery.
If there were no laws on the books regarding marriage – and ever man who wished to marry a woman had to either create their own institution for doing it, redefine marriage to mean something arbitrary or marry in an established religious institution – I submit to you that the following would happen:
...c. Marry in established religious orders with their own body of private church, synagogue or mosque laws governing marriage – thereby making it extremely difficult to actually lead to a situation wherein people would marry who had no intention of staying married, or who thought that the civil laws would somehow sanction their later change of hearts. Would we still have people leaving their spouses? Yes. Would they be able to benefit financially and otherwise from this immoral decision on the basis of civil laws which protect the right to divorce? No. They would, in fact, if ever their vows turned out to be worthless, lose all credibility in said religious community – which would be strengthened by the loss of such elements from their midst.
Thus, by restoring full responsibilities for the regulation of marriage to individuals and churches, we would restore the grand sense of overwhelming obligation that a man and woman ought to feel before what is supposed to be a mighty institution. Surely this serves the development of strong families, secures faith, and ultimately leads to the patriotism of a people who love their country because it gives them the means to be self-governing men and women?
What is "said religious community" and "their country?" A place where homosexual unions have equal dignity with heterosexual unions? Polygamy? Concubinage? If it's not, then Reith needs to come out and say it: the pluralist State has got to go. He apparently doesn't want to venture anywhere near such an outcome, so he switches to a positivist perspective: the State could and should grant this aspect of the right to self-governance. Don't we already have that? Why wouldn't I just secede instead?
Ultimately, it appears Reith is just trying to buffer the State's legitimacy by enlarging its tent. This is, I would say, a very Roman perspective.