Humanae Vitae is online at the Vatican site, and the text highlighted below is © Copyright – Libreria Editrice Vaticana.
26. Among the fruits that ripen if the law of God be resolutely obeyed, the most precious is certainly this, that married couples themselves will often desire to communicate their own experience to others. Thus it comes about that in the fullness of the lay vocation will be included a novel and outstanding form of the apostolate by which, like ministering to like, married couples themselves by the leadership they offer will become apostles to other married couples. And surely among all the forms of the Christian apostolate it is hard to think of one more opportune for the present time. (See Lumen Gentium 35, 41, 40-45; Gaudium et Spes 48-49; Apostolicam Actuositatem 11)
I would also think that God is well able to work grace in a couple when the law is less than “resolutely obeyed.” A significant caution might be a situation in which adherence to the law results in a sort of distance from other couples, especially women and men who have tried but are unable to achieve a high degree of adherence. When it comes to human beings, my sense is that mercy is a more convincing witness than perfection. Or pretending to perfection.