Complexity

Sexuality is very complex neurologically and culturally. Let's say it is something barely understood, and difficult to understand  because there is a strong ideological charge to it.

Now music is also very complex neurologically and culturally, and barely understood as well. It involves the entire brain, not just some musical center of the brain in one particular place, and is tied to memory, emotion, and tons of other things.

So the connection between music and sexuality would have to be super complex as well, because it is the connection between two complex things. So the attempt to connect has to take a kind of short cut; how is this connection understood by people in a specific cultural context? Now, because it is just one  context with limits, we don't have to think about all the complexity involved, we can just ask a simpler question.

So let's take the scene in The Postman Always Rings Twice where Garfield is sitting at a lunch counter. We hear a noise from the ground, and Garfield looks down: the camera cuts to a lipstick rolling on the floor. He picks it up and the camera moves over some shadows of windows frames on the floor and stops at Lana Turner's feet. We don't see her, just the feet, and the camera goes back to a reaction shot. Then we get some sultry jazz-like music to signify "attractive woman." Then the camera shows the entire length of Lana Turner's body.

So the music here just means "entrance of sexy woman" and nothing more. It is a cinematic convention, easily understood and not very deep. We could call it a form of mickey-mousing, where the music just follows what's happening on the screen. It goes together nicely with other cinematic tropes, like the metonymy of the lipstick, the "checking out" of the woman's body beginning with the feet.

If we heard the music in isolation, without the movie scene, we would still have the cultural association with noir music, maybe, but not with the particular characters in the scene. We could use the same music in a neo-noir movie and make the scene about two men, or two women attracted to each other, and it would work equally well.