Today, seeking out images like that puts you on the definite wrong side of a line. A child was harmed to produce that image. You’ll face significant legal and societal consequences if you get caught with it in your possession. Given that you’ve already crossed the line, maybe the next line is not as hard to cross.
Whereas if images like these are decriminalised, folks who look at them don’t have to directly correlate their actions with any specific harm to any specific child, and so that line remains uncrossed.
I think there may be complications around, for example, using inpainting on an image of a specific child, but overall, my approach is more one of harm reduction. Whichever approach leads to fewer real world children being abused is the one to take, and moral outrage should take a back seat to pragmatism if it turns out that decriminalising these images reduces real world offending.
Not sure there’s a good way to test and find out which of us is right though.