The author's main reason for handling Patrick Maloney's announcement in the way he does was probably that he wanted to avoid a dialogue between husband and wife. Mary would be more than likely to argue, to plead, to explain, to ask for explanations, and, of course, to bring up the subject of her expected baby. The impact on Mary would not be as decisive, and the end result would be the same. Patrick wants out of a boring marriage. This fact is essential to the rest of the story. His cold, brutal announcement leads, understandably, to his being clobbered with a frozen leg of lamb.
Mary's behavior before Patrick makes his surprising announcement was intended to illustrate dramatically what it is about her that makes her husband want out of their marriage. She doesn't realize that she is giving him claustrophobia. Any counter-arguments that Mary could make would only soften the impact of Patrick's cold, hard, irrevocable decision to walk out on her. His decision has to be irrevocable in order for her reaction to be, not only credible to the reader, but also sympathetic.