"'Personal values' and the need to respect such are a theme which, over the last twenty years or so, has been considered more and more by writers. In many of their works, even the specifically sexual act has its place assigned, that of serving the 'person' of the married couple. The proper and most profound sense of the exercise of conjugal rights would consist in this, that the union of bodies is the expression and the realization of personal and affective union."
"Articles, chapters, entire books, conferences, especially dealing with the 'technique' of love, are composed to spread these ideas, to illustrate them with advice to the newly married as a guide in matrimony, in order that they may not neglect, through stupidity or a false sense of shame or unfounded scruples, that which God, Who also created natural inclinations, offers them. If from their complete reciprocal gift of husband and wife there results a new life, it is a result which remains outside, or, at the most, on the border of 'personal values'; a result which is not denied, but neither is it desired as the center of marital relations."
Pius agrees with those who say that say that sex should be an "expression and the realization of personal and affective union", but he disagrees with their disproportionate focus on "perfecting" the act. That is, he and the Church believe that sex is more than a pleasurable activity. A perverse focus on pleasure treats children as an unfortunte side effect, rather than the primary end of sexual intercourse.
"Now, the truth is that matrimony, as an institution of nature, in virtue of the Creator's will, has not as a primary and intimate end the personal perfection of the married couple but the procreation and upbringing of a new life. The other ends, inasmuch as they are intended by nature, are not equally primary, much less superior to the primary end, but are essentially subordinated to it. This is true of every marriage, even if no offspring result, just as of every eye it can be said that it is destined and formed to see, even if, in abnormal cases arising from special internal or external conditions, it will never be possible to achieve visual perception."
"It was precisely to end the uncertainties and deviations which threatened to diffuse errors regarding the scale of values of the purposes of matrimony and of their reciprocal relations, that a few years ago (March 10, 1944), We Ourselves drew up a declaration on the order of those ends, pointing out what the very internal structure of the natural disposition reveals. We showed what has been handed down by Christian tradition, what the Supreme Pontiffs have repeatedly taught, and what was then in due measure promulgated by the Code of Canon Law. Not long afterwards, to correct opposing opinions, the Holy See, by a public decree, proclaimed that it could not admit the opinion of some recent authors who denied that the primary end of marriage is the procreation and education of the offspring, or teach that the secondary ends are not essentially subordinated to the primary end, but are on an equal footing and independent of it."
The primary and highest end of sexual intercourse, as the natural manifestationof the supernatural reality of Holy Matrimony,is the begetting of children. This is true even for couple who are infertile through no fault of their own. An eye is still primarily ordered to sight even if its owner is blind. In the case of an infertile couple, there is no fertile period in which to conceive. You can’t abuse what you don’t have. However, I think the Church would argue that an infertile couple should adopt or be foster parents in order to fulfill the procreative aspect of marriage.
"Would this lead, perhaps, to Our denying or diminishing what is good and just in personal values resulting from matrimony and its realization? Certainly not, because the Creator has designed that for the procreation of a new life human beings made of flesh and blood, gifted with soul and heart, shall be called upon as men and not as animals deprived of reason to be the authors of their posterity. It is for this end that the Lord desires the union of husband and wife. Indeed, the Holy Scripture says of God that He created man to His image and He created him male and female, and willed—as is repeatedly affirmed in Holy Writ—that 'a man shall leave mother and father, and shall cleave to his wife: and they shall be two in one flesh'."
"All this is therefore true and desired by God. But, on the other hand, it must not be divorced completely from the primary function of matrimony—the procreation of offspring. Not only the common work of external life, but even all personal enrichment—spiritual and intellectual—all that in married love as such is most spiritual and profound, has been placed by the will of the Creator and of nature at the service of posterity. The perfect married life, of its very nature, also signifies the total devotion of parents to the well-being of their children, and married love in its power and tenderness is itself a condition of the sincerest care of the offspring and the guarantee of its realization."
Again, there are many goods that arise from the sacrament of holy matrimony, and they should not be ignored, but none is greater than the raising of children. Any other arrangements of priorities is disordered.
"To reduce the common life of husband and wife and the conjugal act to a mere organic function for the transmission of seed would be but to convert the domestic hearth, the family sanctuary, into a biological laboratory. Therefore, in Our allocution of September 29, 1949, to the International Congress of Catholic Doctors, We expressly excluded artificial insemination in marriage. The conjugal act, in its natural structure, is a personal action, a simultaneous and immediate cooperation of husband and wife, which by the very nature of the agents and the propriety of the act, is the expression of the reciprocal gift, which, according to Holy Writ, effects the union 'in one flesh'."
"That is much more than the union of two genes, which can be effected even by artificial means, that is, without the natural action of husband and wife. The conjugal act, ordained and desired by nature, is a personal cooperation, to which husband and wife, when contracting marriage, exchange the right."
"Therefore, when this act in its natural form is from the beginning perpetually impossible, the object of the matrimonial contract is essentially vitiated. This is what we said on that occasion: 'Let it not be forgotten: only the procreation of a new life according to the will and the design of the Creator carries with it in a stupendous degree of perfection the intended ends. It is at the same time in conformity with the spiritual and bodily nature and the dignity of the married couple, in conformity with the happy and normal development of the child'."
Humans are more than machines, more than baby factories. Procreation, that is participating in the created order as co-creators, is more than the joining of genes into another human animal. Thus, the conjugal act is the only permissable means of conception.
"To exalt beyond measure, as it is often done today, the generative function, even in the just and moral form of married life, is therefore not only an error and an aberration; it also bears with itself the danger of intellectual and affective error, capable of preventing and stifling good and lofty sentiments, especially in youth which is still without experience and ignorant of life's delusions. For what normal man, healthy in body and soul, would like to belong to the number of those deficient in character and spirit?"
These are good words to remember if someone wholly convinced that any delay of childbearing is wrong should happen to disparage you if you have not yet had a child (or not had "enough" children). Don't judge other people's motives for postponing conception. That is a very different thing, though, than objectively assessing the nature of "serious" and "grave" reasons and thereby troubling someone's conscience, so long as we are careful to not be stumbling blocks to each other.