One of the unexpected features of the Orwellian dystopia is its reactionary attitude about sexuality. At least from the example of similar futuristic societies like that in Brave New World, and from the general fact that the Party in 1984 seeks to overturn the values of the past, one might expect that the old thinking about sex would be overturned as well, especially because chastity is associated with religion, which the Party has destroyed. But in fact the reverse has happened: sexuality is looked upon as evil, absolute monogamy is required, and people are taught that the only rationale for sex is procreation. This is "our duty to the Party," as Winston and Julia derisively note. The Party, or its philosophy Ingsoc, is itself a religion and views sexuality in an even more exaggerated and condemnatory way than the old religions did.
Why is this? Is it, as you suggest, a "divide and conquer" strategy? Yes, but the more specific explanation given by Julia is that because sex makes people feel good, their hostilities are removed, their energies are partly used up, and the Party cannot use people's negative emotions as something to channel into its own agenda. An even simpler reason is that sex is a counterforce to the Party. If men and women are primarily interested in each other, or loyal to each other, they can't be the unthinking zombies the Party wishes to make of them. The energy that normally goes into sexuality is redirected by the Party into hatred—hatred of Goldstein, hatred of whichever of the Three Superstates is the enemy at present, and hatred of others in Oceania who are suspected of Thoughtcrime.