Political scientist Seana Sugrue, in The Meaning of Marriage, starts with the ancient view that the family is pre-political. Marriage and childrearing are not creations of the state, but precede it. Marriage is not a contract created by the market, but precedes it. Having children, she argues, raises marriage above any possibility of being a mere contract to a sacred duty.
Sugrue has this wonderful sentence: "the sexual revolution is to the family what communism is to the market." In the case of the sexual revolution, the state was employed to dismantle the family in the name of greater individual control. Communism, on the other hand, used the state to dismantle the market in the name of greater social control. Both cases, though, ended up making the state more powerful over the institution it was "reforming." The sexual revolution did not just liberate individuals from the family – it left the state in control of defining the family.
Which is why Sugrue argues that establishing legal same-sex marriage would diminish liberty.