What are the rules for sex and marriage among party members in 1984?

We can piece together the following rules for sex and marriage. A Party committee has to approve a marriage, and it won't do so if suspects there is any genuine physical attraction between the two people in question. As for sex, the Party's desire is that it be strictly for procreation. They don't want people, in or out of marriage, to enjoy sex or find any erotic charge in it, because they don't want individuals to form loyalties outside of those to the Party and Big Brother. Winston describes the Party's idea of sex as follows:

Sexual intercourse was to be looked on as a slightly disgusting minor operation, like having an enema.

While hiring a prostitute, inevitably a prole, is not supposed to be done, Winston knows that the Party looks the other way as long as it is merely a one time physical transaction. We learn that having an affair with another Party member, however, is an "unforgivable crime."
Winston's marriage to Katherine provides a good window into a typical Party marriage. Katherine dislikes having sex but insists on having intercourse weekly as her "duty to the Party." Winston finds sexual relations with her odious because of her lack of desire. He also finds the marital relationship unsatisfying in general because Katherine is a completely orthodox Party member who would denounce him as a traitor at the first sign of deviance. This is the kind of lonely marriage the Party wants people to have, which keeps them miserable and focused on the Party.