Papal Encyclicals are traditionally named after their first few words, rather than given a title. (This must give the popes good cause to seriously consider a good strong opening rather something something along the lines of "It has often been observed..." or "In my last encyclical I discussed...")
Pope Paul VI's encyclical issued forty years ago today began: Humanae vitae tradendae munus gravissimum, ex quo coniuges liberam et consciam Deo Creatori tribuunt operam, magnis semper ipsos affecit gaudiis, quae tamen aliquando non paucae difficultates et angustiae sunt secutae.
Roughly speaking: Human life, the passing on of which is one of the gravest responsibilities from which spouses freely and knowingly take on the work of God the Creator, the which always gives them great joy, but also not a few difficulties and a shortage of security.
Given that the Church is often accused of ignoring science and the wonders of human advancement, and that the Church's opposition to birth control is one of the most frequently cited examples of this, it is ironic that the Church's stance on sex and birth control is essentially a restatement of indisputable biological fact: Sex exists, at a biological level, for the creation of children. That's why we have "reproductive organs". Certainly, sex (now that it exists) fills several other purposes as well, but its primary purpose is indisputably reproduction.
The question that faced and continues to face humanity is what to do about this in the face of modern technology which allows us to strip that reproductive function (with degrees of success depending upon the method) out of the sexual act. Starting at the turn of the last century, and with gathering speed with each passing decade, the wider society embraced artificial birth control and the split between sex and human reproduction that this new technology allowed.
The results of this split are still being sorted out, and I suspect that it will be near the end of the new century before we begin to see with any clarity what a society in which sex is only optionally tied to reproduction looks like. With typical progressive zeal, few in the secular realm seemed to imagine at the time (from what I can tell) that anything but good could come from giving people the ability to regulate their fertility with relative certainty through cheap and widely available technology. Surely, people would live just as they had before, but with the ability to make sure they had children only when they were ready to lovingly care for them. How could this be anything but a blessing?
Lots of people with a supposedly rational and naturalistic view of the universe apparently imagined that changing a fundamental element of human physiology (which from a strictly naturalistic point of view must clearly be one of the biggest shapers of human society and culture) would leave existing social structures intact while allowing people to be just a bit more free and joyful in their sexuality. It did not prove to be so. People may not consciously think, "I will get married and be faithful to my spouse because having sex with lots of partners before and outside of marriage might result in having children who would not be raised in a stable family environment." And yet, at a naturalistic level, one of the primary reasons for marriage itself and for chastity before and faithfulness during it is that a stable family environment is required in order to raise the children which naturally result from sex. (Picture, if you can, that there was something we were capable of doing as humans that was as pleasurable and produced as a strong a sense of union as sex -- and yet which never resulted in any consequences other than physical pleasure and emotional closeness. Would society have organized itself in such a way as to require that one shared this act only within monogamous relationships?)
Paul VI, on the other hand, courageously and contrary to the advice of many who had their fingers upon the pulse of the world, reaffirmed in Humanae Vitae that human life is, as God's creation, meant to work a certain way. That sex results in new life is not some accident or medical deficiency to be "cured" by new medical technology, but rather the way in which humans were meant to cooperate in God's creative work. The reproductive potential of intercourse is inherent and essential to it, and to actively remove that potential changes the act in a fundamental way.
The Catholic teaching which he reaffirmed is not, as some critics claim, that women are baby-making machines or that it is immoral to have sex if you can't get pregnant at the present moment. Rather, the Church's acceptance of NFP but rejection of birth control and sterilization amounts to saying: Remain human. Play by the rules we were given. Our bodies are meant to work the way they work. And if you want to avoid having children, you will have to at the very least have less sex.
This is not necessarily easily lived out, even for those of us who accept it, since we cannot help but imbibe the modern ethos in which the sex life has nothing to do with creating human life. Yet this difficulty that we experience is essentially that of living as humans are -- rather than becoming one of that artificially created new race of the optionally fertile. And since we choose to continue living a human life, rather than a sex life, we know roughly what our social institutions and familial relations will continue to look like. We will continue to live as humans have always lived.
How exactly those who have chosen to live a sex life instead of a human life shall eventually sort out their society remains to be seen.