A key element of neoconservatism is its absolute refusal to appeal to a specifically religious justification for any policy position. While they often speak favorably about the benefit of religion, the need for “public square” religion, etc. they are very squeamish about any direct appeals to a particular religion. While they are not rigid secularists in the style of the modern left, they are militant pluralists and universalists, so appeals have to be universal. Hence their obsession with natural law. This is why they went ape over Huckabee who they saw as making direct appeals to Christians as Christians (a sort of evangelical identity politics), and why they loved Romney esp. post his religion speech.
Of course any Christian who is not totally brain-washed by liberal universalism should have no problem opposing same-sex “marriage” explicitly based on the Biblical condemnation of homosexuality. If they are hesitant or unwilling to do so, I think it is appropriate to question their priorities. What are you first? A Christian or a post-Enlightenment liberal universalist? Are you listening First Things?
A Christian should be a christian first in all decisions. And the inigma of Thomas Jefferson comes to mind on these kinds of issues regarding both morality, and seperation of church and state.
It should be a State issue, by referendum. The Federal Government should be reduced to 20% of it’s present size, and those States that vote incorrectly, should expect the wrath of God, and surounding states not recognizing their laws.