From Humanae Vitae:
"[T]he fundamental nature of the marriage act, while uniting husband and wife in the closest intimacy, also renders them capable of generating new life—and this as a result of laws written into the actual nature of man and of woman. And if each of these essential qualities, the unitive and the procreative, is preserved, the use of marriage fully retains its sense of true mutual love and its ordination to the supreme responsibility of parenthood to which man is called. We believe that our contemporaries are particularly capable of seeing that this teaching is in harmony with human reason."
Pope Paul VI had maintained a panel of experts established by Pope John XXIII to consider the morality of contraceptive use. Due partly to media leaks and partly to the cultural milieu, many Catholics and other Americans expected the pope to permit at least limited use of birth control. Instead, Paul confirmed traditional Church teaching that prohibited all forms of artificial birth control (natural methods such as rhythm and sympto-thermal were permitted). The negative reaction of a large number of American Catholics, including priests and theologians, was an unprecedented reception for a papal encyclical in the United States.