Melis Erdur, in 'A Moral Argument Against Moral Realism', asks "whether it makes moral sense to take the dictates of some independent reality to be the ultimate reason why genocide is wrong." (p.7) She continues:
[S]urely, the existence of an independently issued verdict – if there were such a verdict – that genocide is wrong would not be the main or ultimate reason why it is wrong. Genocide is wrong mainly and ultimately because of the pain and suffering and loss that it involves – regardless of whether or not the badness of such suffering and loss is confirmed by an independent reality.
It's a mistake to think that moral realism implies that possession of the mind-independent property of moral wrongness is the "ultimate reason why" an act is wrong, in the ordinary (normative) sense of "reason why". It's a common mistake, though. Matt Bedke writes something similar (though I gather from correspondence that he doesn't really intend it to be read this way) in 'A Menagerie of Duties?': "Is it because they are causing [...] pain that the action normatively matters in the way it does, or because there is some non-natural property or relation at play? Surely the former." (p.197)
As I respond, in my non-naturalism paper (the remainder of this blog post is an extended quote from pp.12-14):
A question like "Why is it wrong to cause gratuitous pain?" can be read in two ways. The most natural reading situates it as a question in first-order normative ethics. This is to ask: What are the wrong-making features of such actions? Which of the (natural) properties in this situation are the normatively significant ones--the ones that do the justifying (or that explain why a certain action is unjustified)? Here the non-naturalist can happily agree with Bedke that it's the causing of pain that's of central normative significance here, and that explains why "the action normatively matters in the way it does."
Non-naturalism is not a first-order normative theory, after all. It instead addresses the (more obscure) metaethical question: What does the wrongness of the action (or the badness of the pain) consist in? Simple answer: The wrongness of the act consists in the act's possessing the property of being wrong! Not a particularly informative answer, perhaps, but it's a central thesis of non-naturalism that normative properties are sui generis. This view eschews the kinds of ambitious metaethical explanations offered by constructivists and others. There is, on this view, no deeper explanation of what wrongness is to be offered. The purely normative properties are bedrock, and the basic normative truths are brutely true. One may or may not like this aspect of the view, but the crucial point for now is just to note that it's compatible with any first-order normative explanations (of which acts "normatively matter" and why). The constitutive sense in which possessing the purely normative property of wrongness is what "makes" wrong acts wrong (in the sense that this is what it is for an act to be wrong) is distinct from, and not in competition with, the normative sense in which certain natural properties are "wrong-making" features.