As Holden sways from one plan to escape the restrictions of society to another, he hits upon an excellent (to his mind at least) idea to just run away and live in nature with Sally and get married. However, it is clear that Sally is less than impressed by Holden's genius, and clearly she sees it for something of what it is: an escape from reality borne out of Holden's frustrations of life. Note how she responds initially to the proposal:
"You can't just do something like that," old Sally sad. She sounded sore as hell.
Clearly she is not as rootless and without connections as Holden feels he is and doesn't feel that she can just up and leave everything behind her in the way that Holden feels he can. She says they are too young and points out the romantic element of Holden's plan, saying that his idea is "fantastic." She believes that mad plans like this should wait until after college. Of course, through the process of arguing about it they begin to hate each other.