There are two themes in Lessing's story that are sometimes separately addressed and at other times seamlessly interact: those themes are sexuality and class and gender barriers.
Tom, the protagonist, wrestles with his ever-increasing sexual desire and his intense insecurity about himself as an attractive or sexually desirable person. He observes a young woman sunbathing and concocts elaborate fantasies about her affection and longing for him. Unfortunately for Tom, the sunbather has no idea what is going on in his head and he is too insecure to approach her in real life.
Class and gender barriers are evident in the way the woman and the workers act (or fail to interact). The woman ignores their comments and cat-calls. She feels objectified; the men feel rejected. Class-wise, the woman has the money and leisure to relax on the rooftop. The men, however, must work at their manual jobs.