Sex in American Cinema:
All Hollywood genres are significantly defined by their treatment of sexual themes and imagery. Musicals are traditionally built on fairy-tale romances and the display of women's bodies; the western often personifies the opposition between nature and culture as a complex heterosexual love story, troubled by an undercurrent of homosexual attraction.
But film noir is an especially provocative case. Film noir plots are defined by their cynical view of corrupted heterosexual love, and by seductive female figures who betray male heroes--and arouse unsettling, ambivalent feelings in the audience. The dialogue in film noir is allusive and elliptical, and often comes across as more dirty-minded than any explicit depiction could be. The lighting characteristically suggests obscure forces lurking in the shadows. The world of film noir is steeped in fetishism: nothing is what it appears to be, and more importantly, nothing is as it should be. The prevailing moral order is both threatened and challenged by the eruption of desires that are usually repressed and unacknowledged. It's not a sunny picture.
For our purposes, perhaps the most interesting aspect of film noir is that it constitutes a counter-tradition within Hollywood film: a self-conscious subversion of the image of American life that the studios generally sought to present. Central to that image is an abiding faith that men and women can find their deepest needs satisfied in romance, courtship and lasting marriage. Film noir proposes instead that sexuality is dangerous, unpredictable and often destructive.
A pretty good introduction to film noir can be found in the Wikipedia article on the topic.
A more extended treatment can be found at John and Stephanie Blaser's Film Noir Studies.