What causes the disintegration of Carlisle's parents' marriage in the novel, They’re Going to Love You?

Not long after Carlisle is born, Robert informs his wife Isabel that he has feelings for his student James. He insists that he does not want to get a divorce and wants to maintain their family structure, but Isabel is too wounded to stay in New York with him. Not long later, however, she decides to go back to the city with Carlisle and to try her marriage again. When she discovers that “Robert is living with James,” Isabel realizes that she must return to Ohio and let Robert and their former life go (39).