Well to be perfectly honest... I'm not that familiar with the american arguments on this subject, but I'm familiar with an argument by european proponents of this type of legislation... And that argument has nothing to do with the harm of the material itself, and this is specifically why their argument simply cannot be defeated.
Because they don't argue that the material itself causes harm. They argue that the material itself IS THE HARM. (sorry for the capitals, no \emph here)
When they look at it from that perspective, there's simply nothing that can change their mind because it doesn't matter what effects the material itself has. It doesn't matter whatever studies you show these people. Their argument is simply that the material itself is the actual damage done, and it must be forbidden.