10 Married love, therefore, requires of husband and wife the full awareness of their obligations in the matter of responsible parenthood, which today, rightly enough, is much insisted upon, but which at the same time should be rightly understood. Thus, we do well to consider responsible parenthood in the light of its varied legitimate and interrelated aspects. With regard to the biological processes, responsible parenthood means an awareness of, and respect for, their proper functions. In the procreative faculty the human mind discerns biological laws that apply to the human person. (9) With regard to man's innate drives and emotions, responsible parenthood means that man's reason and will must exert control over them. With regard to physical, economic, psychological and social conditions, responsible parenthood is exercised by those who prudently and generously decide to have more children, and by those who, for serious reasons and with due respect to moral precepts, decide not to have additional children for either a certain or an indefinite period of time. Responsible parenthood, as we use the term here, has one further essential aspect of paramount importance. It concerns the objective moral order which was established by God, and of which a right conscience is the true interpreter. In a word, the exercise of responsible parenthood requires that husband and wife, keeping a right order of priorities, recognize their own duties toward God, themselves, their families and human society. From this it follows that they are not free to act as they choose in the service of transmitting life, as if it were wholly up to them to decide what is the right course to follow. On the contrary, they are bound to ensure that what they do corresponds to the will of God the Creator. The very nature of marriage and its use makes His will clear, while the constant teaching of the Church spells it out. (10)
Source: Humanae Vitae (Vatican.va)