In Brave New World, society's attitude to sex is extremely liberal. In fact, people are encouraged to have sex with as many people as possible. The state actively discourages people from being monogamous and having relationships or getting married. As we see in chapter 3, for example, women take Pregnancy Substitutes to avoid falling pregnant, and infants are taught erotic play to encourage their liberal attitude towards sex.
Such attitudes towards sex and relationships support the World State's motto, "Community, Identity, Stability." Remember that the state has taken over the act of reproduction. So, instead of having people form attachments and loyalties to one another by getting married and having children, the state ensures that people focus on the needs of the wider community. Moreover, by allowing (and conditioning) people to lead a hedonistic, pleasure-seeking lifestyle, the Controllers know there will be no social unrest, because people are happy. Stability is, therefore, guaranteed.