It is when Julia first passes Winston a note saying, "I love you" that his half-formed rebellion takes concrete shape and form. The couple's first sexual encounter is specifically described as "a blow struck against the Party … a political act." It isn't math or history that strikes that blow, but love. "If they could make me stop loving you, that would be the real betrayal," Winston says. To which Julia replies, "They can't do that … It's the one thing they can't do." Even if you read that as doomed, it's still a fairly romantic bit of dialogue, insisting as it does on the existence of love "in a world where," as Stewart says, "love really doesn't exist anymore."
It turns out, alas, that Julia is wrong; "they" can get inside you. Ciaramella triumphantly quotes the passage in the book where the couple admits that their love has been hollowed out:
“Sometimes,” she said, “they threaten you with something you can’t stand up to, can’t even think about. And then you say, ‘Don’t do it to me, do it to somebody else, do it to So-and-so.’ And perhaps you might pretend, afterwards, that it was only a trick and that you just said it to make them stop and didn’t really mean it. But that isn’t true. At the time when it happens you do mean it. You think there’s no other way of saving yourself, and you’re quite ready to save yourself that way. You want it to happen to the other person. You don’t give a damn what they suffer. All you care about is yourself.”
Ciaramella goes on to argue that, since Winston was doomed as soon as he began to keep a diary at the beginning of the book, the love affair is essentially gratuitous—Winston would have been taken by the thought police anyway. "Diluting this message," of lovelessness and despair, Ciaramella concludes "is a gross affront to the book."
Ciaramella may be right to an extent that the love affair isn't very convincing, but part of the reason for that is that 1984 is an imperfect book. Orwell is able to imagine newspeak and Big Brother and the chief torturer O'Brien with great power, but when he comes to portraying Julia, he flails. She's thoughtless, primitive, interested only in things of the body rather than the mind — "only a rebel from the waist downwards," as Winston calls her. We never really learn why, or feel why, she loves the older, not particularly attractive Winston. We merely know she does because she says so and because, as soon as they meet in private, she starts calling him "dear.” She's just a stereotypical Manic Pixie Dream Girl who's part of Winston's story, not the other way around. So it's not exactly a surprise that she betrays Winston immediately, or that, as O'Brien says, just about licking his lips, "All her rebelliousness, her deceit, her folly, her dirty-mindedness—everything has been burned out of her." None of it was ever really there to begin with.