The 'Mahna Mahna' song was originally written for PORN!
Sorry, everyone's childhoods, the classic Muppet song isn't as innocent as you might've thought. It's one of the best known songs to come out of the Muppets, and it's been re-played and covered a lot of times. However, it wasn't an original composition for Jim Henson's creation, but rather for a 1968 soft-core Italian porn film.
The movie is called "Sweden: Heaven and Hell," and the song is played for a scene in which some Swedish girls go spend some time in a sauna. Check out the clip from the movie below (no worries, it's not NSFW).