"The work of educating in the service of life involves the training of married couples in responsible procreation. In its true meaning, responsible procreation requires couples to be obedient to the Lord's call and to act as faithful interpreters of his plan. This happens when the family is generously open to new lives, and when couples maintain an attitude of openness and service to life, even if, for serious reasons and in respect for the moral law, they choose to avoid a new birth for the time being or indefinitely. The moral law obliges them in every case to control the impulse of instinct and passion, and to respect the biological laws inscribed in their person. It is precisely this respect which makes legitimate, at the service of responsible procreation, the use of natural methods of regulating fertility""