In 1984, Winston suggests that they should break up before they are found out. Julia says no. Why?

In Chapter 7, Winston thinks that he and Julia should break up before their affair is discovered. However, Julia disagrees with him. She tells Winston that she's thought about the consequences and has decided that she will stay with him, regardless of what happens.

Essentially, Julia believes that to rebel is to be human. In the story, the government has greatly restricted the rights of every citizen. Sexual relationships are forbidden and even social discourse is monitored. Each citizen is to be Big Brother's mouthpiece and instrument, and no one is allowed to have autonomy or individual agency. In this light, Julia sees rebellion as a sort of self-preservation. She concludes that Big Brother may be able to force certain words out of her mouth, but he will never be able to take her inner feelings or beliefs away from her.

Julia sees her sexual promiscuity as a sort of self-determinism; she rebels in the sexual realm because this is the only way she can preserve her humanity in light of Big Brother's oppressive influence.

'When you make love you're using up energy; and afterwards you feel happy and don't give a damn for anything. They can't bear you to feel like that. They want you to be bursting with energy all the time. All this marching up and down and cheering and waving flags is simply sex gone sour. If you're happy inside yourself, why should you get excited about Big Brother and the Three-Year Plans and the Two Minutes Hate and all the rest of their bloody rot?

They could not alter your feelings: for that matter you could not alter them yourself, even if you wanted to. They could lay bare in the utmost detail everything that you had done or said or thought; but the inner heart, whose workings were mysterious even to yourself, remained impregnable.

So, Julia disagrees about breaking up with Winston, because it is more important to her to be true to her feelings than to give in to the fear of punishment. She is emotionally invested in her relationship with Winston and is willing to endure the consequences for her choices. For her part, Julia is philosophical about the fate that awaits them.