I remeber seeing some 30s porn animation years ago. Actually, it was quite humorous--not something I would want a kid to see though. Bety Boop herself was supposed to be a hooker. There was one where the character was supposed to be gay. They had this foppish guy swishing around and all the flowers were swishing around--it was hilarious (risque but not actually porn--but you got the message).
Lots of cartoon characters (Dagwood and Blondie, Mutt & Jeff, etc.) were featured years ago in cheap porn knock-offs called "Tijuana Bibles". You can see several examples if you google the term.
I mean, that's what I've been told.