From the Witherspoon Institute website:
Libertarians are being taken in by rhetoric that sounds libertarian but, in fact, will lead to a dramatic shift in the balance of power between the state and civil society, indeed between the state and the natural order itself.
In my previous article, I showed why it is impossible to get the state out of the marriage business. Marriage attaches mothers and fathers to their children and to one another. This is an irreducibly public function. Yet attempting to do the impossible is not harmless. Assigning the state an impossible task amounts to giving it a blank check.
That is because the attempt to privatize marriage will hinder the ability of marriage to perform its essential public function. Nonetheless, children still need to be attached to mothers and fathers somehow. The state will pretend to get out of the marriage business all right, but then the state inevitably will be caught up in the business of defining who counts as a parent. Up until now, that job has been largely left to Mother Nature, with the state simply recording the natural reality of parenthood.