The bulls are potent, the steers, having been castrated, are not. The bulls are center stage in the sport of bull fighting, and are seen as noble and respected. The steers, on the other hand, exist to serve the bulls. They are put in the corrals with the bulls to
"...keep them from fighting, and the bulls tear in at the steers and the steers run around like old maids trying to quiet them down."
To be a steer is not a good thing; sometimes the bulls "go right after them and kill them." The steers' sole purpose is to "quiet down the bulls and keep them from breaking their horns against the stone walls, or goring each other." As Bill sardonically notes,
"It must be swell being a steer."
In Jake's group, Jake, having been rendered impotent in the war, is the steer. He and Brett seem to have a close tie, but because he cannot consumate their relationship, he is relegated to the role of the steer, forever trying to calm the men who can. Mike and perhaps Robert Cohn are the bulls, vying for the favors of the female, Brett Ashley, although I believe Robert Cohn can also be considered a steer, because when forced to confront Mike, he is sorely lacking in resolve and effectiveness. This relationship among Jake, Brett, Mike, and Robert Cohn, the steer(s), the female, and the bull(s), is representative of the values of the time among the "Lost Generation" because it illustrates that immediate gratification and physical satisfaction are more important than love. It is Jake whom Brett loves, or seems to love, but since he cannot have sex with her, she constantly seeks the company of other men more potent than him. Jake, on his part, can do nothing more than act the part of the steer, looking after Brett while others vie for and win her favors (Chapter 13).