As Julia observes, the Party polices sexual relationships because it realizes that the hysteria caused by sexual frustration can be harnessed into war fever and leader-worship. Because of this, when Winston and Julia make love they think of it as a political act, "a blow struck against the Party." The sadistic fantasies Winston has about Julia before they begin their affair indicate the strong link between sexual repression and violence. The red sash Julia wears and her voluptuous appearance arouses feelings of hatred and resentment that only dissipate when he learns that he can possess her physically.
Another reason that the Party restricts sexual behavior is that sexual desire competes with loyalty to the State: after Winston makes love with Julia, he realizes that it is "the force that would tear the Party to pieces." In place of heterosexual love, the Party substitutes leader-worship and patriotic feeling: thus, when Winston betrays Julia under torture, he learns to revere O'Brien and worship Big Brother.
Sex, Love, and Loyalty ThemeTracker
Sex, Love, and Loyalty Quotes in 1984
And then—no, it was not relief, only hope, a tiny fragment of hope. Too late, perhaps too late. But he had suddenly understood that in the whole world there was just one person to whom he could transfer his punishment—one body that he could thrust between himself and the rats. And he was shouting frantically, over and over.
“Do it to Julia! Do it to Julia! Not me! Julia! I don’t care what you do to her. Tear her face off, strip her to the bones. Not me! Julia! Not me!”
"All you care about is yourself," he echoed.
"And after that, you don't feel the same towards the other person any longer." — "No," he said, "you don't feel the same."