Dick, in his very clinical and rational way, agrees that he must not continue to see Nicole, just as he very rationally and clinically told Rosemary why they could not have an affair. The fact that the reader knows that Dick and Nicole marry foreshadows the fact that his clinical denial of Rosemary will be aborted as well.
Far more important, though, are Dick's dreams and what he recognizes a marriage to Nicole would mean. Fitzgerald's ability to present the totality of a life in the shortest phrase is nowhere more apparent than when he writes of Dick's time as a reclusive scholar: "Most of us have a favorite, a heroic period, in our lives and that was Dick Diver's." In this period of his life, and in all of them leading up to the events of Book 1, Dick is presented as a driven and ambitious scholar, not the socialite that we have come to know. In his discussions with the other doctors, it is clearly observed that if Dick marries Nicole, she will become his life's work. He will be both her husband and her doctor and that will consume him. With this in mind, in line with the logic of his life, Dick agrees to terminate his relationship with her, although he has begun to have feelings for her.