One of the main things I've noticed thus far in "The Sun Also Rises" is the flailing masculinity that all the male characters exhibit. During the aftermath of WWI, there was a radical re-evaluation of what it meant to be masculine and the issues that the male characters in this book deal with represent a small picture of the larger cultural issue. The pre-war ideal of the brave, noble soldier had little relevance in the context of brutal trench warfare that characterized the war as survival depended far more upon luck than upon bravery. This is what caused the traditional notions of what it meant to be a man to be undermined by the realities of the war and it is the character of Jake that best embodies these cultural changes. The war renders his manhood (ability to reproduce) useless because of injury which causes him to experience a nagging sense of inadequacy. 

While Jake’s condition is the certainly the most straightforward example of weakened masculinity in the book, he is definitely not alone in his "loss of manliness". All of the veterans feel insecure in their manhood and while Hemingway does not state this explicitly, it is clearly seen when Jake and the others consistently mock Cohn. They only target him when they feel that he is performing "unmanly" actions and it is their need to humiliate him that reveals their deeper issues. They appear to be coping with their fears of being weak and emasculated by criticizing the weakness they see in him -- which, unfortunately, makes them appear to be even more lost than he (Cohn) is.

An additional juxtaposition to this theme in the book is the character of Brett. In many ways, she is more “manly” than the men in the book. Brett refers to herself as a “chap,” and wears a short, mannish haircut as well as possessing a masculine name. She is strong, independent and embodies all of the traditional masculine cliches that the veterans lack.