You have made a reasonable point that in child pornography, there is the additional problem of child (what do they call them? actors? stars? models? rapees?) being utilized in the production. The Japanese have solved that particular problem with child rapee CGI's.
But there is the additional problem that, as I said in #35, the First Amendment was arguably intended to protect all controversial speech and publishing, i.e. the conveyance of ideas. Cognitive content. That did not include, even then, sedition, which is a form of treason; and I would argue it did not include porn, which is arousal-based (which is to say, comparable to the purveying of a harmful drug) --- and not communication-based.
I think it could be objectively defined, too. And on a more objective basis than, "I don't know how to define it, but I know it when I see it."
I don't think the "prurient interest" criterion is utterly useless, but going a step further, surely some University can monitor the sexual arousal patterns of experimental subject-viewers, program that basic info into a computer, and thereafter use the computer make a reasonable assessment of whether the material in question is physiological-arousal oriented rather than intellectual-cognitive oriented.
You can theorize that the first amendment doesn't protect speech that is intended to arouse, you can even invent a "cognitive test" but neither are supported by a textual or historical analysis of the 1st amendment. Further even if you could demonstrably prove without a shadow of a doubt that it is not protected speech, that does not give government the legitimate power to ban it.
Please note that your argument is directly analogous to that of the liberals anti 2nd amendment arguments of "The founders never intended the 2nd amendment to protect assault weapons". Further it forwards the premise that government has any power except for those specifically curtailed, which while wildly popular amongst democrats, turns constitution principle on it's head.
The only legal way to ban pornography is via constitutional amendment and even that would be dubious considering the 1st amendment which does not grant a right but recognizes a natural right.