If I were writing this essay, my thesis would read something like this:
In the society of 1984, sex and intimacy are not supposed to go together. Sex, of course, must occur for children to be born. Beyond that, it is not supposed to happen. Intimacy is not supposed to happen at all. Therefore, Winston and Julia use sex and intimacy as a way of expressing their rebellion against the Party.
To write the essay, I would then provide some examples. I would talk about the Junior Anti-Sex League and about Winston's wife's attitudes towards sex. I would talk about Julia's affairs with Party members. These would show the official attitudes towards sex and the way that sex could be subversive. Finally, I would talk about why the Party would want to discourage sex and intimacy. I would say that it was because they do not want people to form strong attachments outside the framework of the Party. They do not want people to have other loyalites.