Several thoughts:

a) Isn't the solution to qualify the "libertarian argument" by limiting it's scope to "any terms that don't break the law"? (Of course "libertarian" is a poor adjective choice since a legal contract very much relies on a powerful state backing the enforcement of any breach to mean anything--the concept of a libertarian contract is an oxymoron.)

b) What do suspected ulterior motives on the part of those advancing the "libertarian argument" or the fact that sincere libertarians are a fringe minority have to do with the argument's logical validity?

c) In reality, in the case of marriage, the state isn't merely a neutral enforcer but a party to the contract <em>as well as the contract's enforcer</em>. That is to say, the married couple's rights and responsibilities with respect to the state are modified by the contract. In particular, the way they are taxed changes; so may citizenship or residency status. And it also affects a couple's relationship to fourth parties--e.g., if Bob is married to Ron, Ron may, in some cases, be held liable for Bob's debts.