In public policy terms, the Catholic critique of “gay marriage” reflects the Catholic idea of the just state. Rightly understood, marriage is one of those social institutions that exist “prior” to the state: prior in terms of time (marriage existed before the state), and prior in terms of the deep truths embedded in the human condition. A just state thus recognizes the givenness of marriage and seeks to protect and nurture this basic social institution.
By contrast, a state that asserts the authority to redefine “marriage” has stepped beyond the boundaries of its competence. And if that boundary-crossing is set in constitutional or legal concrete, it opens up a Pandora's box of undesirable results. For if the state can decree that two men or two women can make a “marriage,” why not one man and two women? Two women and two men? These are not paranoid fantasies; the case for polyandry and polygamy is now being mounted in prestigious law journals.