I rub elbows with libertarians (note the lower-case "l," signifying a political philosophy rather than political party) quite frequently. Their most common rejoinder to the traditional conservative argument (that marriage should be recognized for what it is — between one man and one woman) is that the state should be removed from marriage entirely and enforce private contracts only.
Girgis, Anderson, and George have a powerful rejoinder to that argument in their new book What Is Marriage? The first sentence of the excerpt below summarizes their reasoning nicely — regardless of what libertarians would prefer, removing the government from marriage is impossible as a practical goal:
Abolishing civil marriage is practically impossible. Strike the word ‘marriage’ from the law, and the state will still license, and attach duties and benefits to, certain bonds. Abolish these forward-looking forms of regulation, and they will only be replaced by messier, retroactive regulation — of disputes over property, custody, visitation, and child support. What the state once did by efficient legal presumptions, it will then do by burdensome case-by-case assignments of parental (especially paternal) responsibilities.
The state will only discharge these tasks more or less efficiently — that is, less or more intrusively. It can’t escape them. Why not? Because the public functions of marriage — both to require and to empower parents (especially fathers) to care for their children and each other — require a society-wide coordination. It is not enough if, say, a particular religion presumes a man’s paternity of his wife’s children, or recognizes his rights and duties toward their mother; or if the man and his wife contract to carry out certain tasks. For private institutions can bind only their own; private contracts bind only those who are party to them. A major function of marriage law is to bind all third parties (schools, adoption agencies, summer camps, hospitals; friends, relatives, and strangers) presumptively to treat a man as father of his wife’s children, husbands and wives as entitled to certain privileges and sexually off-limits, and so on. This only the state can do with any consistency.
But more than inevitable or necessary, it is fitting that the state should do this. Consider a comparison. Why don’t even the strictest libertarians decry traffic laws? Firstly, orderly traffic protects health and promotes efficiency, two great goods. Second, these goods are common in two senses: private efforts cannot adequately secure them, and yet failure to secure them has very public consequences. It is not as if we would have had the same (or even just slightly less) safety and efficiency of travel if people just did as they pleased, some stopping only at red lights and others only at green. Nor would damage from the resulting accidents (and slower shipments, etc.) be limited to those responsible for causing it. To ensure safe and efficient travel at all, and to limit harm to third parties, we need legal coordination. Indeed, it is no stretch to say that the state owes its citizens to keep minimum security and order: to these we have a right. Finally, unlike private associations, the state can secure these goods, without intolerable side effects. All this makes it appropriate for the state to set our traffic laws.
In an essay solely on political theory, we might argue the details, but here we can extract from this example a widely acceptable rule: If something would serve an important good, if people have a right to it, if private groups cannot secure it well, everyone suffers if it is lost, and the state can secure it without undue cost, then the state may step in — and should.
All these conditions are met in the case of marriage.
Be sure to pick up a copy of What Is Marriage? for one of the most convincing defenses of man-woman marriage around.