Why is Winston shocked by Julia's careless for nudity in his dream?

 In Winston’s dream, Julia pulls off her coveralls and anti-sex sash. Winston isn’t so much sexually aroused by this more than he is surprised by her utter disregard for the party. She rips off her clothes like an animal unadulterated by party puritanism. It is an act of primal passion, which Winston believes is the only way to destroy the party.