I have recently come to the (re)realization that bishops are the authoritative teaching body of the Church. As such, it is their responsibility to properly and effectively teach such sticky subjects as the regulation of births. However, those teachings must be in accord with the Bishop of Rome and magisterium of the Church, so I still think there is merit in exploring the relevant papal documents. Let us then continue by hearing the thoughts of Pope Pius XII.
I had thought that Pius XII had written an encyclical about contraception. As it turns out, the only statements he made about the subject were in in various allocutions (addresses) to associations of doctors and the like. These don't carry nearly the same weight as encyclicals and are certainly not infallible. An exploration of the doctrinal authority of papal allocutions can be found here, but I cannot vouch for its accuracy. Nevertheless, Paul VI quotes from these addresses extensively in Humane Vitae, thus lending some of theauthority of an encyclical. I searched for the texts of these addresses and only found the 1951 Address to Midwives on the Nature of Their Profession and the 1958 Address to Officers and Representatives of the Associations for Large Families-of Rome and of Italy. If anyone knows where I might find the rest of them, I'd be indebted. Anyhow, here's the address to midwives.
"When one thinks of this admirable collaboration of the parents, of nature and of God, from which is born a new human being in the image and likeness of God, how can the precious contribution which you give to such a work be not appreciated? The heroic mother of the Machabees admonished her children: 'I know not how you were formed in my womb, for I neither gave you breath, nor soul, nor life, neither did I frame the limbs of every one of you. But the Creator of the world that formed the nativity of men . . ..' [Machabees 7:22-23a]"
"Therefore, he who approaches this cradle of life's origin and exercises his action in one way or another must know the order which the Creator wishes maintained and the laws which govern it. For here it is not a case of purely physical or biological laws which blind forces and irrational agents obey, but of laws whose execution and effects are entrusted to the voluntary and free cooperation of man."
"This order, fixed by the supreme intelligence, is directed to the purpose willed by the Creator. It embraces the exterior work of man and the internal assent of his free will; it implies action and dutiful omission. Nature places at man's disposal the concatenation of the causes from which will rise a new human life, it is for man to release its loving force, for nature to develop its course and lead it to its completion. When man has completed his part and placed in action the marvelous evolution of life, his duty is to respect its progress in a religious manner, a duty which forbids him to arrest nature's work or halt its natural development."
Put more succinctly, the sexual act is inherently one of procreation. It is a participation in God's work as Creator. This work is not to be performed at the sole discretion of an individual or couple.
"You, more than others, can appreciate and realize what human life is in itself, and what it is worth in the eyes of sane reason, before your moral conscience, before civil society, before the Church and, above all, what it is worth in the eyes of God…The child is 'man,' even if he be not yet born, in the same degree and by the same title as his mother."
"Besides, every human being, even the child in the womb, has the right to life directly from God and not from his parents, not from any society or human authority. Therefore, there is no man, no human authority, no science, no 'indication' at all—whether it be medical, eugenic, social, economic, or moral—that may offer or give a valid judicial title for a direct deliberate disposal of an innocent human life, that is, a disposal which aims at its destruction, whether as an end in itself or as a means to achieve the end, perhaps in no way at all illicit. Thus, for example, to save the life of the mother is a very noble act; but the direct killing of the child as a means to such an end is illicit. The direct destruction of so-called 'useless lives,' already born or still in the womb, practiced extensively a few years ago, can in no wise be justified[…]"
Like Pius XI before him, Pius XII felt the need to reemphasize the Church's strict prohibition against abortion. No reason, "whether it be medical, eugenic, social, economic, or moral", not even the life of the mother, can justify the "direct deliberate disposal of an innocent human life". I think it's sad that such an obvious point is so often ignored by Catholics that it must be restated ad nauseum. Skipping ahead a bit:
"Our Predecessor, Pius XI, of happy memory, in his Encyclical Casti Connubii, of December 31, 1930, once again solemnly proclaimed the fundamental law of the conjugal act and conjugal relations: that every attempt of either husband or wife in the performance of the conjugal act or in the development of its natural consequences which aims at depriving it of its inherent force and hinders the procreation of new life is immoral; and that no 'indication' or need can convert an act which is intrinsically immoral into a moral and lawful one."
This precept is in full force today, as it was in the past, and so it will be in the future also, and always, because it is not a simple human whim, but the expression of a natural and divine law."
He restates Pius XI's assertion that "no 'indication' or need can convert an act which is intrinsically immoral into a moral and lawful one". The intrinsically immoral act referred to is the use of artificial borth control. Historically speaking (and peeking ahead at the rst of this address), weknow that Pius XII approved of limited use of natural means of birth control, so it seems that he did not believe such natural methods to be intrinsically immoral. We'll get to that later, though.
"It would be more than a mere lack of readiness in the service of life if an attack made by man were to concern not only a single act but should affect the organism itself to deprive it, by means of sterilization, of the faculty of procreating a new life. Here, too, you have a clear rule in the Church's teaching to guide your behavior both interiorly and exteriorly. Direct sterilization— that is, whose aim tends as a means or as an end at making procreation impossible—is a grave violation of the moral law and therefore unlawful. Not even public authority has any right, under the pretext of any 'indication' whatsoever, to permit it, and less still to prescribe it or to have it used to the detriment of innocent human beings."
"This principle is already proclaimed in the above mentioned Encyclical of Pius XI on marriage. Thus when ten years or so ago sterilization came to be more widely applied, the Holy See saw the necessity of expressly and publicly declaring that direct sterilization, either perpetual or temporary, in either the male or the female, is unlawful according to natural law, from which, as you well know, not even the Church has the power to dispense."
Sterilization is forbidden – no ifs, ands, or buts about it. As far as the Church is concerned, not even civil authorities have the right to force it upon anyone (though one has to wonder how the Church would practically oppose such an inducement).