To add to allyson's point, the other benefit for the party in controlling/ extinguishing the sexuality of its members is one of loyalty. If sex is not joyful (or, better still, if it is repulsive), then people do not form strong emotional bonds with one another. Their passion can then be more readily converted into love (for the party) and hatred (for Goldstein and whatever enemy they are currently pretending to be at war with). With intimacy comes a shift in priorities. The first responsibility is to our immediate loves, then after that we will be responsible to our work and our country. Big Brother needs to keep people more in love with Him than with their families. Killing the sex instinct is one way to do this. A child conceived out of a distasteful but necessary duty will not be received by the parents in the same way as a child conceived out of lust or love. The ownership of the child, both physically and emotionally, shifts away from the parents toward the party. This also allows for the lines of loyalty to be redrawn. If a child is created specifically for the party (and not for the joy of creation), then the loyalty of both parent and child can be directed to the party. No love, no family.