And now you're in court strenuously arguing that you weren't sexually gratified by the photo of your kid in the tub.

Obviously most people are sensible most of the time but sometimes they are not.

More than that. CSAM is evidence of abuse. Hence the "A".

And nudity is not required.

CSAM has a meaning identical to child porn but doesn't make that meaning explicit. Drawn or generated depictions of child nudity can be considered CSAM in some jurisdictions.

"CSAM isn’t pornography—it’s evidence of criminal exploitation of kids."

That's from RAINN, the US's largest anti-sexual violence organisation.

Yep. Germany is very very strict for example. Even textual descriptions fall under that law.