i think it's the same as with any reactionary response: "i did not want to see this (harmless) thing because it makes me remember CSA is real and now i have to think about what you're doing and its (non-)relation to CSA, and for making me have to think i should be allowed to call the cops on you."

they cannot make any reasonable case that the art could somehow cause CSA, so they are left with no moral explanation of why the thing they saw that disgusted them is harmful. refusing to deal with their own disgust, they then offload their feelings into empty moralism and label it degenerate art.