From ‘Gay Marriage and Creational Realism‘ (part 5 in my 6 part series on nominalism vs. realism)
For the nominalist, in order for God to be truly free and all-powerful, the categories by which our moral and material lives are ordered must be the result of God’s disposing will and not rooted in structures antecedent to His will (i.e., the fixities of God’s nature or the inherent patterns of creation). The nominalist will thus find it difficult to speak of things being “fitting” or “rightly ordered” in any sense more general than, or prior to, God’s pedestrian commands….
Unfortunately, the impasse of communication that persists in the “gay marriage” debate has left some Christian thinkers suspecting that genuine dialogue with unbelievers about the meaning of marriage is impossible. The thinking tends to run something like this: if someone doesn’t share our Christian worldview, there isn’t much we can appeal to when defending traditional marriage. Moreover, why would it even make sense for the other side to listen to us given that they don’t share the worldview that gives rise to our understanding of marriage in the first place?…
If the Christian understanding of marriage arises from the raw command of an omnipotent God arbitrarily constituting the world in a certain way that might just of easily have been otherwise, then I agree that there is little we can say about the moral constitution of the world to those who do not share our theocentric worldview. On the other hand, if we are realists then we believe that God’s commands about sexual ethics flow out of the teleological directedness intrinsic to creation itself. Under the realist scheme of things, it becomes possible to appeal to unbelievers on the basis of that ordering without needing to invoke explicitly Biblical arguments.