You could make a multitude of arguments against that perspective, but at least there is a conclusive reason for legal restrictions.
> What is the criteria for this?
My criteria would be victims suffering personally from the generated material.
The "no harm" argument only really applies if victims and their social bubble never find out about the material (but that did happen, sometimes intentionally, in many cases).
You could make the same argument that a hidden camera in a locker room never causes any harm as long as it stays undetected; that is not very convincing to me.