What is Joy's explanation for sleeping with Philip when he was in Genoa?

Joy explains that she and John, her husband, met when very young; both were sexually inexperienced and moved in together fairly quickly, so that when they were married, nothing really changed. He was a good father and husband but she always felt as if something were missing; she never felt desire for or desired by John passionately.When Philip arrived, she could feel his desire from across the room and she was overwhelmed with the giddiness of being wanted so desperately.