In this particular scene, Montag calls his wife's friends, Mrs. Bowles and Mrs. Phelps, "monsters" after listening to their conversation about their home lives. Mrs. Bowles has had two children by Caesarian section because she does not want to experience the "agony" of childbirth. Moreover, she claims to have only had children because she feels duty-bound to do so, not because she desires motherhood:
The world must reproduce, you know, the race must go on.
Mrs. Phelps has not had children because she believes they are "ruinous." Even though Mrs. Bowles has children, she clearly does not possess any maternal instinct. She parents by putting her children in school for nine days out of ten, and leaving them to watch the parlor walls.
This cold and unloving attitude prompts Montag to call the women "monsters" amid his astonishment at their way of life.