The local scanning would've been enabled only if you would've uploaded the photos to iCloud anyway.
To complete your crappy analogy: You had already agreed to send the information of everything contained in your apartment to a company, and now you're getting into a hissy-fit when they are sending a robot visit your house with a list of known child pornography photos and checking if you have any?
There weren't any "hard decisions" it wasn't a "this has a naked person in it" -scanner. It wasn't an "abuse checked" it was specifically created to find known CP.
It checked your photos against a NeuralHash of KNOWN child pornography. So the only chance your sauna photos would've been flagged if they were included in a set of CP photos distributed widely enough so that they're added to the CSAM database.
And nobody claimed the system would magically cure the world of grooming, where did you get that idea from? Although the current filters in iMessage might help if the abuser is stupid enough to use that for grooming.