Strawman.  Neither R&R nor I say that sin is "some ideal bad form."  Nor does calling certain acts "objectively wrong" or "objectively sinful" require positing some Platonic Form of the BAAAAD.  You don;t seem to grasp that "sin," as it is usually understood, is simply the combination of an act in disobedience to God's will, with knowledge that it is disobedient and the intent to commit the act.  

There is no more "mental construct" here than in saying that you committed an act in defiance of your mother's will, knowing it was defiant and intending to do the act.   The only difference is that in one case, we're talking about God's will rather than your mother's.  And by the way, since, when you come down to it, God is more real than your mother, sin against God is even LESS a "cognitive construct" than disobeying your mom.