In this scenario, the man and woman are presumably fertile and able to have children. And they have no grave reason to avoid procreation. They simply prefer their lives to be free from children. They are able, but they do not wish to, raise a family.
In Catholic teaching, the primary purpose of marriage is generatio et educatio prolis, the generation and education of offspring, also called the procreation and raising of children [Pope Pius XII, Address to Midwives, n. 66]
If either the man, or the woman, or both, marry with the specific and firm resolve to never have children, even though they are able, then the marriage is not valid. If the man and woman are baptized Christians, their attempted union is not a valid Christian Sacrament of holy Matrimony. If one or both of the parties are unbaptized, the union is not a valid natural marriage. For they do not intend the primary purpose of marriage.
Similarly, if either or both parties do not intend a lifelong union, the marriage is not valid. For marriage is a lifelong union of man and woman, ordered toward the procreation and raising of children.
For any Sacrament, the person or persons must intend to do what the Church does by that Sacrament. A couple who do not intend a lifelong commitment, or who directly intend to prevent the marriage from procreating and raising children, do not have the proper intention needed for a valid Sacrament.
This raises the question as to whether a “Josephite” marriage is valid. Certainly, from the time of the exchange of consent at the wedding ceremony, to the consummation of the marriage, the usual case is that the couple has a ratum tantum marriage. Then, after consummation, the marriage is ratum et consummatum. But can a couple remain continent throughout the entire marriage? For a grave reason, maybe. But without a grave reason, I don’t see how such a union can be valid, since the couple are rejecting the primary purpose of holy matrimony.
If the couple are elderly, and so they cannot conceive, they still have a valid marriage, as long as they engage in the natural act that is open to life. What if the couple have a grave reason not to procreate, for example, if they realize, prior to the marriage, that any children would have severe birth defects? It seems to me that they should not marry, and they cannot fulfill or even attempt to fulfill the primary purpose of marriage. The elderly couple at least is open to the procreative finality, whereas this latter couple intend a lifelong union absent procreation.
There are some unanswered questions in this area of theology. I can’t say that all my answers on this topic are dogma. We await further clarification from the Magisterium. But if a couple directly intend not to have children, I don’t see how their marriage could be valid.
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