Language changes. “porn” has been used to express “visually stimulating imagery” and “simulacra of the real activity” for quite some time. There is nothing dumb about appending porn to the SFW subreddits, because there is no better expression (*) to succintly express that those subs are not made for deep/meaningful discussion.

(*) Maybe it could work with “obsession” or “fetish”, but the pearl-clutching crowd would also criticize the sexual connotation.