In "Fahrenheit 451", why do Mildred's friends react as they do when Montag reads them a poem?

The scene you refer to occurs in the second part of the book, "The Sieve and the Sand".  Montag reads the poem, "Dover Beach", to Mildred's friends, Mrs. Phelps and Mrs. Bowles.  The two friends are women just like Mildred - self-centered and empty-headed.  Mrs. Phelps says that her husband was recalled by the Army to fight in the war.  She says she is sure he'll be home in a week, but her words ring somewhat hollow.  The reader has been given hints that there is going to be a cataclysmic war soon, so the reader knows Mr. Phelps won't come back home.  Both women realize, as Montag gets the book, that what Montag is about to do is illegal and they are very uncomfortable having to sit there and watch this illegal behavior.  Mrs. Phelps begins to cry as Montag reads, though.  The poem is about a man going off to war and what happens to him, so she is thinking about her husband, even if the connection between the poem's words and her husband is a subconscious connection.  Mrs. Bowles even seems to realize that there is a connection.  When she sees Mrs. Phelps crying, she becomes angry.  Her anger could be partly from guilt in realizing the truth of the poem and the truth of the civilization's situation.  Both women reflect what Beatty had told Montag earlier in the story - that books make people think and sometimes thinking makes people realize uncomfortable truths.