The poem is structured as one stanza written in free verse and built around contrasting the female narrator's relationship to the apartment with her male lover's. The title and content of the poem raise the question of what the "sin" is. Is it two unmarried people living together (the traditional meaning of living in sin), or the narrator's violation of her own needs to service this relationship?
For her lover, the apartment is a vehicle or backdrop for doing what he wants—idly play the piano, telling her it is out of tune, looking in the mirror, rubbing his beard, going for cigarettes. For the speaker, the space is fraught with anxiety. She sees it as a place she needs to clean—it won't dust itself, the food won't put itself away, and the mouse lurks as an unwanted presence. While her lover goes out, she makes the bed, "dust[s] the tabletop," lets the coffeepot "boil over," a metaphor for her own anger. That she resents her domestic burdens is suggested, too, by the idea that it is not until evening that she is "back in love again."
The enjambments—a technique in which the end of poetic line is not the end of a thought—keep us moving, as the speaker does, from thought to thought and worry to worry, highlighting that she is the one who has to deal with maintaining the living space that her male partner inhabits so carelessly.