Why did the party need to destroy the "sex instinct"

The party knows that the sex instinct is the hardest to control, especially for men. Women control access to this so the party wishes to brainwash them. The act of sex between two outer party members can develop loyalties outside of party doctrine. The junior anti-sex league, for example, indoctrinates young women to reject sex for pleasure. Sex is to be an unwanted act only necessary for procreation (by orders of the party with people chosen by the party.)Julia, of course, is heavily involved in this as a way to mask her secret activities.