The speaker sets two moods side by side. Inside his chest there is "a thin rain," a quiet, lingering sadness that never quite becomes a storm. Outside, in the disco, young women "bought" him, and he becomes "light for night," something bright and pleasant to have around until morning. The image suggests he is valued for his glow rather than for anything deeper.
The title, "Spiritual teacher," makes the last two lines land with dry humor. His Zen, the calm and wisdom he might once have offered as a guide, turns out to be "useful in bed." Spiritual practice becomes a skill that helps him be a better companion for the night. The tone is wry and self-mocking, with a hint of loneliness underneath. He doesn't sound proud or ashamed, just amused and a little tired, aware of how far his teaching has drifted from its purpose. The short lines keep everything light, which makes the sadness easy to miss.