From a comment on Reddit:
If libertarians believe that government allowing same-sex marriage is in fact an expansion of government, they should also support the abolition of government-allowed opposite-sex marriage in an attempt to reduce the imposition of government into our lives. I do not see any explanation in this article as to why endorsing the so-called “aberrant” sexual practices is any more imposing than supporting “normative” sexual practices.
The issue here is not of endorsement but of proper management of national resources. In this case that resource is new citizens. New citizens can only come about through the sexual activities of a man and a woman. Because that is the only way new citizens are brought about the government has a vested interest in making sure that new citizens are properly cared for by its progenitors (everyone has a mother and a father) and raised in a stable home and given the tools to become well adjusted citizens. Not that any of hat is impossible for a homosexual person, but it has statistically been shown that a child who enjoys the benefit of their biological parents are better off than those who haven’t. In short, we make laws and policies based on the ideal standard, meaning a mother and father and children in a stable home, and not the exceptions to the rule.
So since the government has no vested interest in affirming the individual sources of happiness of its citizens and since children are the only real reason marriage is treated differently than any other contractual situation, it stands to reason that any attempt to redefine the institution of marriage is inherently counterproductive to the goal of producing well adjusted citizens.