Saint Augustine of Hippo, a great theologian and bishop of the fourth century, came up with important aspects of marriage known in Latin as bona (goods), the ends or fruits of marriage. First and foremost, Christian marriage is for the good of the couple (both the husband and the wife). This is called the bonum coniugum in Latin. It is the very essence of the sacrament of matrimony.
Catholic theology teaches that a valid sacrament of Christian marriage depends totally on the intention of both the bride and the groom on their wedding day, specifically at the moment they exchange consent (by either saying “I do” or by saying the vows together). Both must intend to enter a permanent, a faithful, and, God-willing, a fruitful union, otherwise there will be no bonum coniugum and therefore, no marriage. Saint Augustine referred to these three intentions as bonum sacramenti (permanence), bonum fidei (faithfulness), and bonum prolis (offspring).
Children are not the only purpose of marriage, as some claim Catholicism maintains. However, procreation is one of the three essential aspects. It is crucial to point out here that it is the intention, desire, and willingness to have children that is essential. Married couples who are biologically unable to have children are still very much married and in a valid sacrament. Sterility is not an impediment to the sacrament but a dispensation can be given to individuals who are impotent and thus unable to consummate the marriage. It is only those who purposely frustrate the overall good of the couple by intentionally excluding one or more of the three necessary goods (e.g., by habitual use of contraception or nontherapeutic sterilization) that render the union invalid.
Certainly understandable, but nonetheless still sinful, are immoral ways of procuring children, such as in vitro fertilization, egg and sperm donation and implantations, and surrogate mothers. These scientific processes involve immoral means, such as masturbation, selection of the “best” fertilized egg and abortion of the others, freezing fertilized eggs, and infidelity. Children must be brought forth in a human way between a husband and a wife, not in a laboratory. Fertility can be aided by science, such as in vivo fertilization, which involves no immoral means. When infertility is experienced by the couple, it is a cross that should be carried bravely. Though not required to, the couple is encouraged to consider adoption, a selfless act of love on the part of the married couple.
Being open to the possibility of children does not mean having as many babies as is biologically possible, either. Natural family planning is a moral and ethical way to use morally acceptable means to guide the number and frequency of pregnancies. If one or both spouses intend never to have children, this is as serious a defect as would be the intention not to be faithful or the intention not to enter a permanent relationship. Prenuptial agreements are indications that someone may not be intending a permanent bond, which invalidates the union. Equally invalidating would be those situations when people intend never to have kids lest they ruin their chances for a promising career.
In 1968, Pope Paul VI issued his famous papal encyclical, Humanae Vitae, in which he teaches that Christian marriage and especially the marital act of sexual intercourse are sacred and holy symbols of the unitive (love) and the procreative (life) dimensions which make up the foundation of the sacrament of Marriage.
Marriage is seen as one of the two sacraments of vocation, the other being Holy Orders. Through this sacrament, the couple promises to enter a permanent bond that can only be dissolved by death, to be always faithful (in other words, exclude any other sexual partners), and to be fruitful or to be open to the possibility of children if God wills it. The last requirement comes from the Old Testament (Genesis 1:28), when God said to Adam and Eve, “Be fruitful and multiply.” In the Catholic sense, marriage is the fertile ground not only to populate the earth, but eventually to populate heaven—our ultimate goal in the road of life.