Your contrasting "objective sin" with sin as a construct is confusing and not entirely correct.  

First of all, there IS "objective sin," or what St. Thomas calls "material sin."  It is not the kind of sin that brings moral guilt, and is not what most people think of when they think of sin.  

Secondly, you are right to say that sin as we usually mean it --- "actual" sin --- is subjective, in that it requires the actor's knowledge of the wrongness of the act and the intent to commit it.  Your phrasing, however, invites the conclusion that nothing is objectively wrong, which I don't think you really want to say.

Finally, to say there is "only a construct of sin" seems to say that sin does not exist except as a creation of the human mind.  But physical acts are real, and when a real act violates God's will --- which is also real --- there is nothing about the fact of violation that reduces the sin to a mere human construct.  If you disagree, pinpoint the error.