This argument falls flat regarding synthetically-produced CSAM and CSAM-adjacent material (no human beings exploited in its creation; in fact, one could argue that creation of synthetic CSAM depresses the market for child-exploiting CSAM), so I wouldn't be surprised if the US can't find a way in their legal structure to ban such material; their protections against obscenity in general tend to be weaker than other nations, and if the content is merely obscene and not generated by harming a minor, it's harder to argue it should be banned (as opposed to shunned for being odious).
In the US, there is a legal distinction between child pornography and child obscenity. Both are criminal, and exceptions to the 1st Amendment – but, the first is much easier to prove in court. In the 2002 case of Ashcroft v Free Speech Coalition [0] SCOTUS ruled that (under the 1st Amendment), child pornography only included material made with real children – so written text, drawings, or CGI/AI-generated children are not legally child pornography in the US. In the US, child pornography is only images/video [1] of real children. If someone uses editing software or AI to transform an innocent image/video of a real child into a pornographic one, that is also child pornography. But an image/video of a completely virtual child, that doesn't (non-coincidentally) look like any identifiable real child, is not child pornography in the US.
What most people don't seem to know, is that while a virtual child can't be criminal child pornography in the US, it still can be criminal child obscenity – which is rarely prosecuted, and much harder to prove in court, but if they succeed, can still result in a lengthy prison term. In 2021, a Texas man was sentenced to 40 years in prison over a bunch of stories and drawings of child sexual abuse. [2] (Given the man is in his 60s, that's effectively a life sentence, he's probably going to die in prison.) If someone can get 40 years in prison for stories and drawings, there is no reason in principle why someone could not end up in federal prison for AI-generated images too, under 18 USC 1466A. [3]
[0] https://en.wikipedia.org/wiki/Ashcroft_v._Free_Speech_Coalit...
[1] maybe audio too? I'm not sure about that
[2] https://www.justice.gov/opa/pr/texas-man-sentenced-40-years-...
The US on the other hand is a different story. In 2005 one man was convicted to 20 years imprisonment for hentai. In 2016 a man was convicted to 10 years imprisonment for owning a coloring book.
Wikipedia has a few more cases in the article "Legal status of fictional child pornography".