The sex instinct was another area of human life that the Party sought to kill or at least distort. Only the “proles,” the so-called lower class people who did not actually qualify to belong to the Party, were allowed to have sexual instincts. Among Party members all love and eroticism was removed from the sex act. Marriages were permitted, but they had to be officially approved and were to be undertaken for the sole purpose of begetting children who would grow up to be responsible Party members. Permission would be denied if the couple showed any signs of being physically attracted to each other. Ideally the Party would prefer complete celibacy, which would mean that men and women would forge fewer bonds of individual loyalties and children could be produced artificially. Failing this, it tried to present the sex act as a rather disgusting preliminary necessity.
Winston would not have minded remaining celibate, but Katharine insisted they go through the process at fixed intervals to “fulfill their duty to the Party” by producing a child.
"Sexual intercourse was to be looked on as a slightly disgusting minor operation, like having an enema." Part 1, Chapter 6, pg. 69
"What was more important was that sexual privation induced hysteria, which was desirable because it could be transformed into war fever and leader worship." Part 2, Chapter 3, pg. 134
All love and eroticism are banned among Party members. Marriage stops at the point where it is purely a "union" for "production of Party members", in a blunt sense. Sex becomes a disgusting but necessary duty along with the marriage.