Adrienne Rich uses a female speaker to show the loss of Romanticism in a premarital cohabitation that the speaker thought would sustain itself, but now she is having to clean up some of the mess and put away some minor demons.
The apartment is a metonym for the couple's relationship. The speaker thought it would keep itself, but now--over time--she has come resigned to the fact that it is work, that much of the idealism about the new apartment (relationship) has worn off. She's not condemning marriage or men, but she's having to become the domesticated woman that she had not envisioned she'd be. The minor demons (her mother, or society) are bothering her in that she can hear them say, "I told you so." She feels social expectations weighing on her and the pangs of guilt ("Living in Sin") are felt the morning after. Indeed, relationships--married or not--are messy, and hard work, especially in the morning.
The poem reminds me of the scene in Act III of Romeo and Juliet, after the newly married couple has spent their first night together. The scene takes place in the morning, like this one. Juliet wants Romeo to leave before he is discovered; she is the Realist who worries for his safety. Romeo, the Romantic, would rather be caught and killed than leave his love. The honeymoon was over; it was the last time they would see each other alive.
This poem is obviously less tragic, but the dualities are the same, except the female speaker was once a Romantic (or she still is, but only at night), and now she is a Realist (in the morning). The couples' nights are filled with passion, as evidenced by the cheese and three empty wine bottles. But, by morning, the romance has worn off: the piano is out of tune; she's got to dust the table with a towel; he can't wait to leave. The milkman's early morning wakeup call is a reality check.