Sex in BNW is used as a form of release, an outlet, and as entertainment--not as a form of procreation. In the BNW sex and promiscuity is encouraged and programmed into all of its inhabitants. Children run around and play erotic games, adults have the feelies, and promiscuity is celebrated. The only people capable of providing the embryos for fertilization are freemartins--women whose sexual apparatus is left untouched and pristine. Huxley has created an inverse world from the way we traditionally see sex. Certainly, our times have become more and more sexually liberal, but think of what an effect his novel's portrayal of sexuality must have had on the readership of his day...
In Brave New World sex has been decoupled from love and childbearing. It is engaged in purely for fun. Children are encouraged to start exploring sex at a young age, such as six or seven, and the old world (i.e. Huxley's time period, the late 1920s/early 1930s) in which people put off sex all the way through their teens until "they were over 20 years old" is greeted with shock.
People are told about sexual mores in the "old world":
For a very long period before the time of Our Ford, and even for some generations afterwards, erotic play between children had been regarded as abnormal (there was a roar of laughter); and not only abnormal, actually immoral (no!): and had therefore been rigorously suppressed. A look of astonished incredulity appeared on the faces of his listeners. Poor little kids not allowed to amuse themselves? They could not believe it.
People have sex purely for physical pleasure rather than as an expression of emotional intimacy with another human being, because emotional bonding with others is seen as destabilizing to the narcotized world the Controllers have constructed. People are strongly conditioned against the messy intimacy of falling in love.
As for childbirth, babies are "decanted" in "hatcheries" where their lot in life is genetically predetermined. Women use birth control and abortion to avoid pregnancy. Sex is another symbol of the sterilized, stunted lives people lead in the new society.