Stanley v. Georgia considered the question from the right to privacy side, not the first amendment. The relevant cases are https://en.wikipedia.org/wiki/Ashcroft_v._Free_Speech_Coalit... and United States v. Williams https://en.wikipedia.org/wiki/PROTECT_Act_of_2003#Supreme_Co...

This is an extremely important point to understand. At face value, it can feel like CSAM should be protected speech, however repulsive. There are no laws against gore videos, though one might argue that death is worse than SA. However, we have substantial empirical evidence that CSAM directly contributes to offending behavior.

We also acknowledge that participating in pornography requires consent, and that the continued distribution of nonconsensual pornography constitutes a continuing crime against the unwilling subject. Because children have zero legal capacity to consent, CSAM is de facto illegal.

It misses the point to think that CSAM is illegal because it is "obscene". It isn't illegal because it's disgusting; it's illegal because it's egregiously harmful to children. It's like thinking the bad thing a murderer did was make a mess.