I thought it was about Harford's journey using ultra-pretty settings and cinematography through a dangerous seedy world, of which he learns little as he cannot really enter it all, as if the prettiness protects him, and he remains an outsider, where the world is just full of potential danger and seediness. The sex activity is removed from him by luck or circumstance and he just floats on top of a sea of filth unharmed.
But I also thought it was basically crap, too much of an art film, not Kubrick's real strength.