
I think the problem there is that, for many years, nobody bothered to explain to him exactly why child porn is bad.
Most people observe that everyone else thinks it’s bad and don’t question it any further. That’s not good enough for Stallman, though, and for good reason: expecting him to unquestioningly bow to peer pressure is an insult to his intelligence.
Someone did eventually explain the problem to Stallman. I don’t know what exactly was explained, but my guess is that Stallman was told that child porn is non-consensual and therefore violates the child’s privacy, similar to how revenge porn violates the subject’s privacy. At any rate, after that discussion took place, Stallman did an about-face on the subject, and is now opposed to child porn like anyone else.
Moral of the story: taboos and peer pressure bad; logic and education good.
We kinda do need him, though. Very few people are as intensely principled as he is on the subject of computing freedom, and without him anchoring the Overton window, there’s nothing stopping the Bill Gateses of this world from moving it.