Aside from the fact that we make law appeasing religious zealots regularly, you miss the point. As well, what christians are asking that the state get out of the marriage business? If anything, it's the opposite. Religious persons of all flavors want state support. So let's not.

Take religious marriage away from the state. Nobody is harmed. Nobody is prevented from getting married by the state because the state simply doesn't do marriage. Civil union is a secular institution, a nice safe sandbox for the state.