Over the years, I've found plenty of areas where I disagree with the libertarian view. Too much of what they propose is pure anarchy, stripping government down far beyond the minimum necessary to function. So it goes with most libertarian arguments for removing state influence from marriage or eliminating all restrictions as to who is able to participate in it.
At the core, marriage is a contract. In any other contract, the state is necessarily involved should one or more parties to the contract fail to fulfil their obligations. Proposing the elimination of government influence in marriage contracts is to say that might makes right. This law of the jungle approach is both unpalatable and impractical. This is especially so when there is no written contract in place. There must be a definition as to what defaults apply in these cases. There is a strong case that since the state will be dragged into the conflict resolution process, we must accept that government will be involved in marriages.
In the midst of this, some present the argument that marriage should be available to any two loving adults who wish to form this contract. However, if we test this law against the extreme of incest, it falls flat. In fact, you're very unlikely to find any advocates of same-sex marriage asking that we repeal any incest laws. So we have to ask: why do we accept this restriction and do not consider it an infringement upon equal protection? I see only one of two possibilities: the overwhelming majority thinks it has a sufficient "ick" factor to be outlawed or we accept that reproductive viability is a key part of endorsing relationships. In either event, we're likely to accept that government has the proper authority to declare some couples as unable to get married.
We must ask, then, what conditions we will set upon who can and cannot enter into a marriage contract. In order to do so, we need to look at the purpose of marriage. Modern sensibilities tell us that self-fulfillment is a primary goal of marriage. While that may be a positive effect of marriage, such selfish goals are hardly the purpose. No, the purpose of marriage is and always has been to create a stable environment for the raising of children with both a male and female parent. Any other benefit that comes from marriage is a nice side effect, but it is not the goal. Because we place very high societal value upon the successful raising of children, we provide numerous benefits to married couples to promote that end. Is it, then, unfair to not extend the same benefits to same-sex couples?
Some of these things are already available via other legal instruments. For instance, a living will can easily grant inheritance and hospital visitation rights. Tax breaks are available to anyone with dependents, so it is already a universal benefit. And what of health insurance? That's a matter of negotiation between employer and employee. So what benefit is left that is being denied by restricted marriage to heterosexual couples outside of a certain degree of consanguinity? I'm hard-pressed to think of any.
Given this, I've only been able to come to one conclusion as to what the goal is: to force societal recognition of same-sex relationships as healthy and normal using the force of law. Despite protestations that such things will not occur, I have seen little evidence that once marriage is attained, the lawsuits to force churches, businesses and individuals to similarly provide this recognition aren't far behind. It's telling that the same group who says that it won't happen is also unwilling to codify such protections into law. It betrays their real purpose and motivation.