In nadsat, breasts are called "groodies." All the girls and women (other than the ten-year-olds) have them. They seem to make the boys wild with desire. Well, at least this is true for Alex, since he's the only source we have, and he does talk incessantly about them hanging out of every woman's shirt, and their pink nipples waving hello at him, he is thereby caused to have the urge to do the old in-out-in-out. So, they very obviously symbolize femininity and sexuality; and, in Alex's case, a rape-to-come.