From Julia's perspective, why does the Party want to extinguish sexual activity in 1984? Part 2, Chapter 3.

In this chapter, Julia and Winston discuss the reasons why the Party wants to make people stop having sex.

Julia believes that the reason for this is that the party wants people to have pent-up, unreleased sexual tension that will cause them to become hysterical.  She says that the Party could then take this hysteria and redirect it so that people put all that energy into loving Big Brother and hating the enemies of Oceania.

Julia says that people who are engaged in a sexual relationship will be too happy.  They will not want to bring themselves to the fever pitch of hatred needed for the Two Minute Hate, for example.

So, sex is bad, in the eyes of the Party (Julia says) because it makes people happy.  The Party wants them frustrated so that they will be more able to hate.  The Party wants them to have no one to love so they will love only Big Brother.