While it is repugnant, whether or not it is immoral is another matter. If it is immoral, it likely deserves being criminalized, whereas if it is not immoral, democratic liberal ideals suggest that the law should protect such manufacture from unjust censure instead.
The case against filmed CP is straightforward: minors are harmed in the process. But what is being harmed in generated imagery? If nothing is harmed, what is the basis for it being immoral?