Why does Julia belong to the junior anti sex league even after she becomes involve with Winston?

Julia belongs to the Junior Anti-Sex League as a form of rebellion and subversion against the Party's oppressive control over sexuality. While she engages in a romantic relationship with Winston, her participation in the League allows her to outwardly conform to Party expectations while secretly defying them. This duality highlights her resistance to the Party's ideology, as she seeks personal freedom and pleasure in a society that seeks to suppress individual desires. Ultimately, her membership is a strategic facade that reflects her complex relationship with the Party's oppressive rules.