While parts of the movie are whimsical, much of it is more adult in tone. The scenes involving the sex shop, for example. I don’t think that one sequence was gratuitous at all. The sex sequence in Delicatessen might be a better example.
Earlier in the movie, Tyler Durden (I think) had taken a job as a projectionist, purely to insert such images into kids movies. It’s a meta joke that they do the same thing later on in Fight Club itself.