Marriage licenses are ENTIRELY government related. Here, test it yourself: Pick any young religious couple you want. Have them perform ceremonies in every church, synagogue, mosque, or temple in the state and see how "married" they are when they file their taxes. All they did was hold a religious ceremony and no number of priests or pastors is going to amount to anything until a government official files the paperwork. Do whatever you want in your church. That's the ceremony. Marriage is a legal institution and license dispensed by the government to promote social stability. So no, it's absolutely as un-libertarian as I can conceive to be opposed to same sex marriage.