To be clear, OP didn't say "porn" on Lemmy, they said "adult art". There are contexts in which nudity is not pornographic, but I assume it's easier to just ban it all and not take any risks, than to judge the artistic merit of each upload before deciding whether it's too over the top.
The inverse problem exists for creators on yt who make animations that are not kid friendly. Google's bots love to tag them as for children, and thus demonitize them.