An interesting question. I would say the answer is found in her response to Winston's recitation of his limits: "I don't care." Julia chose Winston because she likes sex and because he is unexceptional. If he were handsome and powerful, there would be a more obvious reason for her to rebel, but she likes the sex itself, and desire that is free and without anchor is, as Winston notes, a blow against the party. Her desire shows that she is a rebel, but perhaps a bit shallow and certainly not an intellectual.