The Church believes that a marriage must contain four essential elements, that it must be free consent of both parties, that their gift to each other is total, that they will be faithful to each other until death, and that they are open to life. Formal refusal to accept or practise one of them would make the marriage defective, and is considered grounds for annulment. If one or more parties refuse to agree to one of these elements before the marriage, the priest will not be allowed to marry them.
Without going into the details of the other components, let me just focus on the necessity of procreation.
In the very beginning of creation, when God created the first man and woman, His command was that “the two may become one flesh” and that they may be “fruitful and multiply”. These two “unitive” and “procreative” natures have always been integral to marriage. It is written into the very fabric of man and woman.
It is unitive since despite the vast difference between man and woman, they are made to complement each other, both physically and otherwise. Just look at our bodies and it is abundantly clear what our Creator meant. The one flesh union is said to point to the perfect union between Christ and His Church when all shall be consummated on the Last Day.
It is procreative, since the very nature of the conjugal act is designed to conceive life. Conception from the conjugal act is the consequence of a conscious act and a sign of health, not a defect. Every modern invention to impede conception has proven to do more injury to the conjugal act than to enhance it: chemical (or oral) contraception does violence to the woman’s body, as if her fertility is a disease that needs to be beat down even with harmful chemicals; and mechanical contraception (like condoms and IUDs) puts a barrier between the two parties, contradicting the very act of union they are supposed to be entering into, thus making the whole thing a lie. Besides, oral contraception does not actually prevent ovulation (to do so it would require a dosage so powerful that it would kill the woman). What it instead does is that failing to prevent ovulation, it stops the production of progesterone, which in turns stop the uterus from building up the necessary lining for the conceived embryo to implant. This means that the pill does not stop conception, but forces the conceived child to be automatically aborted.
A couple getting married, who are decidedly against having children, therefore, is setting themselves up to not wanting children. They are setting themselves up to practise contraception, which either puts them at risks of aborting their children even unknowingly, or from the outset putting a barrier to their unitive act.
And how can a contraceptive marriage be free, since I am not willing To embrace you unless part of your healthy self—your fertility—is counted out? And how can it be total, since I can give everything except my fertility? And how can it be faithful, when for the sake of our idea of marriage, I rather make every act of union a lie, or taking chemicals that can abort our children?
I am sorry that the above seems blunt and rude, but there is no sugar-coating it. That people do not know this does not make it any prettier. I had worked with many couples in marriage preparation, and dancing around the issue never worked. They are too blind to see it in their emotional excitement. But the truth that contraceptive marriage is dangerous must be told. It is destructive. It is selfish. It is not marriage as God created it to be. And the Church cannot affirm what God does not affirm, because it is harmful to her children.
Please, ask your daughter and her fiancé to sit down with their priest and talk this over. If they are simply afraid to have children right away, the Church does not ask married couples to have children irresponsibly. The practice of natural family planning methods like the Billings’ ovulation method and the Creighton Model allow couples to plan responsibly without trespassing against the dignity of marriage, and in fact when practised correctly, often enhances the marital relation.