Living in Sin is a newly married young woman's analysis of the reality of her marriage as compared with her dreams for it. In the fantasies she held prior to the marriage, daily chores like cleaning and cooking and disagreements with her husband were not present. The reality of her wedded life, she is discovering with dismay, involves bugs and dust and disruptive noises outside the apartment and dissatisfied comments from her husband within.
She is in love with her husband in spite of it all, but is not in love with the circumstances of her life. Her feeling of being trapped in disagreeable surroundings and activities would only be heightened by the addition of a newborn and all the disruptions any baby causes.
Perhaps Rich is suggesting that those whose expectations are too high are punished for their sins of not being willing to accept whatever comes. If the child was conceived during the time of fantasy before the wedding, Rich could be commenting that the couple is now paying for their sin of premarital intercourse.