Winston describes his relationship with Julia as “a blow struck against the Party.” Explain why their relationship is such a political threat?

Winston and Julia's relationship is forbidden, the fact that they're getting away with it (for the time being) is a direct blow against Party rules. According to Winston, primal desires like sex is what is going to destroy the party rather than politics.

In the old days, he thought, a man looked at a girl’s body and saw that it was desirable, and that was the end of the story. But you could not have pure love or pure lust nowadays. No emotion was pure, because everything was mixed up with fear and hatred. Their embrace had been a battle, the climax a victory. It was a blow struck against the Party. It was a political act.