A libertine can be defined as someone who pursues sensual pleasures and lacks moral restraints. Julia certainly appears to indulge her physical appetites to the full, unlike her lover Winston who is intellectual. She enjoys sex and claims to have had many affairs with other Party members. In this way she fits the definition of a libertine. However we should also look at her motives for behaving as she does. She is lashing out at the Party which aims to repress all natural human spontaneity and instinct. It can be said that the restrictions placed on natural instincts, most of all sex, lead to her rebellion; she takes her pleasures wherever she can find them. This applies not just to the actual act of lovemaking but also her interest in feminine accessories such as make-up and perfume which are also forbidden by the Party.
In view of the Party's inhuman restrictions Julia's seemingly wanton behaviour is very understandable. It is presented in a positive light, as the mark of humanity against the dehumanizing tendencies of Party rule. Winston certainly thinks of it in these terms and admires her for rebelling in this fashion. She herself seems to care rather less about the political implications of her behaviour, however.