What does Wonston find exciting about his dream about Julia tearing off her clothes?

Ironically it is not so much the sexual aspect of Julia ripping off her clothes that excites Winston. It is the fact that she rips off the sash of the anti-sex league that most arouses him. Winston thinks it is the primal urges of people that will ultimately destroy the party. The tearing of the sash becomes a metaphor for liberation. The Party can no longer control desires like sex and these uninhibited desires, Winston hopes, will fracture the party's control over people.