[GovernmentShrinker]

Why do the dictators in Orwell's fiction and in real-life China so fear sex?

Same reason the leaders of authoritarian and fundamentalist religions fear sex.

[WackyKat]

But the deeper reason for anti-sex attitudes, as Orwell understood it, is because sex focuses one's attention on one's own happiness and away from service to the political regime

You could insert "faith" for "political regime" and this sentence would remain true.

Fundamentalists, whether political or religious, really hate sex and are obsessed with controlling it because they sense, correctly, that erotic feelings and acts are a direct challenge to the control of mind and behavior which they wish to exert over people