What is the role of relationships and intimacy in 1984? What specific function does the Party's directive on sexual interaction serve?
Close relationships, particularly sexual ones, are forbidden by the Party because they create divided loyalties. In the Party's world, the citizen must have absolute and undivided faith and trust in Big Brother and the Party alone. As O'Brien explains to Winston (Part III, ch. 3),
We have cut the links between child and parent, and between man and man, and between man and woman. No one dares trust a wife or a child or a friend any longer. But in the future there will be no wives and no friends .... Procreation will be an annual formality like the renewal of a ration card. We shall abolish the orgasm. Our neurologists are at work upon it now. There will be no loyalty, except loyalty towards the Party. There will be no love, except the love of Big Brother.
With all competing loyalties abolished, the citizen will be able to become exactly what the Party wishes him to be at any moment.
Forbidding sexual relationships and any real intimacy also has the effect of frustrating Party members due to the blocking of a natural drive, and building up a store of violent emotion that can be directed by the Party into other channels. As Julia's intuition tells her, "sexual privation induced hysteria, which was desirable because it could be transformed into war-fever and hero-worship" (Part II, ch. 3) Thus, sexual intimacy is seen by the Party as robbing it of energy that it should by rights be able to utilize, a type of political rebellion.
Relationships and sexual intimacy are anathema to the Party. The Party wants to keep Party members sexually frustrated, isolated, and entirely devoted to the state. It wants its people to experience only the emotions of hate, anger, fear, and triumph. As we learn from Winston's ruminations, the Party also wants to characterize sexual intimacy solely as "duty" for the Party; in other words, sex is simply a way to have children and is entirely devoid of any other nurturing function.
Winston's relationship with Julia is therefore key to the novel, for it is through this relationship that he becomes fully human. Before he falls in love with Julia, he sees her from afar—and is sexually attracted to her—but he sees her wearing the red chastity sash and has emotions of anger towards her. He imagines hurting and killing her. The Party has successfully filled him with violent hate. After he enters into a caring relationship with her, however, he changes and becomes loving and more humane. Their relationship is the ultimate transgression: they care about each other more than the state, and for a brief time develop a life outside of the Party's political conventions. This is why it becomes so important to O'Brien that they betray each other: they cannot be fully reclaimed by the state while they still love one another.
Relationships and intimacy are not allowed, and the Party's directives on sexual interaction serves to advance the influence of the Party.
The Party does everything it can to destroy feelings of empathy and loyalty between individuals because all such attention should be focused on the Party itself. Recognizing the potential of the sex instinct to "create a world of its own which (is) outside the Party's control, the Party controls relationships with more than Puritanical zeal. Sexual urges open the door to love and caring about others, which is counterproductive and even dangerous to the Party's goals. Youth is conditioned to actively campaign for chastity, and promiscuity is anathema; marriages must be approved by the Party on an individual basis, and exist solely for the purpose of procreation, thus ensuring more future members for the Party. The children produced in the context of marriage are brainwashed at an early age to spy on their parents and report any deviance in their behavior, thus perpetuating and strengthening a system in which intimacy is impossible, no one trusts anyone else, and every action or thought is observed and controlled - all for the advancement of the Party (Section 2, Chapter 3).