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

thinks,
earlier in the novel, that: 

The Party was trying to kill
the sex instinct, or, if it could not be killed, then to distort it and dirty it. He did not
know why this was so, but it seemed natural that it should be so. 

Later, he thinks:

 The sexual act, successfully
performed, was rebellion. Desire was thoughtcrime.

, who,
according to Winston, relates everything back to her own sexuality, penetrates to the heart of
the Party's hatred of sex in a specific way that Winston doesn't. In other words, he recognizes
that her insights are more acute that his own. As the above quotes show, he knows the Party
wants to destroy the sexual instinct and he knows too that, in the Party's mind, desire,
including, sexual desire, is a thoughtcrime, but his insights don't go much deeper than that
recognition. Julia, however, has a pragmatic grasp of the Party's thinking: While the Party does
resent anything that diverts attention from it and is outside of its control, she realizes that
the Party also wants to harness sexual energy to its own agenda. It doesn't want anything that
could be used for it to go to waste:

When you make love
youre using up energy; and afterwards you feel happy and dont give a damn for anything. They
cant bear you to feel like that. They want you to be bursting with energy all the time. All this
marching up and down and cheering and waving flags is simply sex gone sour. If youre happy
inside yourself, why should you get excited about Big Brother and the Three-Year Plans and the
Two Minutes Hate and all the rest of their bloody rot?

Julia understands that people who are sexually satisfied aren't going to possess
maximum pent-up energy to offer to the state. What she partially misses, but will become crystal
clear after she and Winston are arrested, is that aside from any practical value in suppressing
people's sexual energy and diverting it to nationalism, the members of the ruling elite want
people to be unhappy because that is the only way they know they are exerting maximum power over
the populace: the elite can only be sure people are doing with the Party wants, and not what
they as little people  want, if the little people are miserable.