My solution would to put the burden on the defense to prove that the imagery is faked. It’s the only way I can think of to not leave a hole in the law large enough to drive a truck through. If that also stamps out the production of I Can’t Believe It’s Not Child-Porn, so be it.
How could simulated underage porn possibly be illegal? If an image doesn’t represent a specific person, all you would have to do is write “This person is actually 18” (or for that matter, 300 years old) on the image, and it would be impossible to disprove.
To answer the OP, I once heard it put this way (paraphrasing): “It’s only a God-given right if it does not infringe on somebody else’s God-given right.”
So a person has the right or freedom to “express” whatever the damn well please, as long as that “expression” doesn’t infringe on somebody else’s rights or freedoms. CP fails that test, as it infringes on at least two of the victim’s rights: privacy & safety.