Apparently, Brett and Jake are madly in love. Except that Jake can't have sex, and Brett doesn't love him enough to give up on having sex forever. Love is a strange and terrifying concept. Anyway, Brett's fixation on sexual relations seems to be her own special way of coping with her life's lack of direction. She likes to run around and have lots of meaningless intercourse, and it makes her feel like her life has meaning, but in reality, she recognizes that this behavior isn't fulfilling at all, and in reality, she's "been so miserable" (p. 32). So this looks like another case of people trying to physically "move" away from their problems. First we had Cohn who wanted to move to South America, and then there was Jake who constantly hops from bar to bar, and now there's Brett who jumps from significant male other to the next. This sense of "movement" seems to be a recurring idea throughout the novel. That's a motif, yeah?...yeah. Furthermore, it appears that Brett's very presence seems to frustrate Jake, as is seen on page 38: "To hell with Brett. To hell with you, Lady Ashley." She seems to serve as a constant reminder to Jake that he just can't do the nasty like he once could.

Perhaps it's just the vicodin talking, but I really could not come up with a relevant picture.
Edit: I'm not actually a vicodin addict; I got my wisdom teeth pulled two days ago.