Reflection on July 11, 1984
https://www.ewtn.com/library/PAPALDOC/jp2tb113.htm
We move from looking at Scripture to looking at the encyclical Humane Vitae. SJPII looks at the structure of a very small part (only a couple of lines) of the entire encyclical, but very important, and controversial, lines. "The Church teaches as absolutely required that in any use whatever of marriage there must be no impairment of its natural capacity to procreate human life … This particular doctrine, often expounded by the Magisterium of the Church, is based on the inseparable connection, established by God, which man on his own initiative may not break, between the unitive significance and the procreative significance which are both inherent to the marriage act … The reason is that the marriage act, because of its fundamental structure, while it unites husband and wife in the closest intimacy, also brings into operation laws written into the actual nature of man and of woman for the generation of new life". These few lines give a great over view of the Theology of the Body and SJPII understands that what he has done is dig into this encyclicals instruction and further proven its Truth through his reflections.
I was not around for the release of Humane Vitae, but it is my understanding that leading up to its release it was the general consensus that the Pope was going to release an encyclical that changed Church teaching and allowed contraception or at least became more flexible on the subject. Its release, preaching just the opposite, was met with hostility from a world that saw the Catholic teaching on these types of issues as behind the times. I have mentioned it before in relation to the issue of divorce, remarriage, gay marriage, etc, that the world either thinks or hopes that the Catholic teaching is going to change. I am not sure if a serious and diligent reading of Theology of the Body leaves any doubt that Catholic teaching on marriage is firm and grounded in the teachings of Christ and will not change.