True, but what if the couple knows of the impotence and still is willing to go through with it? Who’s business is it but theirs and God’s?
Others may correct me if I do not succeed in explaining it clearly here, but …
Marriage is a natural state and right of all men and women. However, Christ elevated this natural reality to the level of a sacrament, in which the man and woman who are married become channels of God’s grace for one another, allowing them to share in God’s divine life and grow in holiness as they participate in His creative activity. So, because Christian marriage is “something more” than a natural marriage, i.e. a sacrament which can help the couple and their children grow closer to God and receive salvation, it is very much the Church’s “business” to whom and how the sacrament is administered. One of the reasons that Christ founded His Church was to safeguard and administer the sacraments.
Now, in the Catholic understanding of the sacrament, it is actually the bride and the groom who are the ministers of the sacrament to one another, not the priest. So, because one of the Church’s functions is to ensure the correct administration of the sacraments which Christ entrusted to her, the Church must first determine that the couple who wishes to enter into marriage are a) free to do so, b) **capable **of doing so and c) actually do so according to the form which the Church has laid down for the administration of the sacrament. A defect in any of these three affects either the validity of the sacrament (in other words, whether or not the sacrament actually took place) or its licitness (whether it took place according to the correct form for its administration).
This is why a priest must ordinarily be present when the couple exchanges their vows: he is the Church’s official witness to the validity and licitness of the sacrament (there are others–the “best man” and “maid of honour”, for example).
This brings us to the sexual act. In the Catholic understanding of human sexuality within marriage, sex is not a “bonus” of marriage, it is a fundamental part the reason that marriage exists at all, for the reasons stated in earlier posts: drawing the couple together and strengthening the bond of love between them, and the transmission of human life. It both creates human life and, ideally, provides the loving and protective environment which the child needs to grow and develop correctly as a person.
So, the problem of impotency is this: however much the impotent party may love his partner in the romantic and emotional sense, he is simply not capable of bringing to completion an act which is a fundamental part of the marriage bond, as well as the
right of his spouse.
This is why, if a man knew that he was impotent prior to marrying and concealed this fact from his wife, she would have grounds for an annulment once this fact came to light.
If he were to disclose it to the priest, but not to his wife, the priest would not be able to act as a witness because he would know that it was a farce … a serious defect exists which affects the validity of the sacrament.
If the couple both knew it beforehand and concealed this fact from the priest, the marriage ceremony would likewise be a farce, but the priest would not know it and would be acting in good faith as the Church’s representative in the ceremony. It would appear to be a valid and licit marriage but, in fact, would not be.
Now, this is the part that I would appreciate the help of someone who knows Canon Law well with, because I may not be correct here

: if the couple were to take this deceitful course of action with the intention of seeking treatment for the problem of the impotent party afterwards, it would still be an invalid and illicit sacrament. Should the husband at some point in the future overcome his impotency and be able to complete the marital act, for the couple to be considered sacramentally married, they would have to approach the Church and confess their deception. The first “marriage” would then be annulled and the couple would have to exchange vows once more.
I think that’s correct, but again, let’s have one of the Canon Law aficionados/experts clear that up, please! Canon Law was never my principal interest, so I may have missed something along the way …
This is all very hypothetical, of course, and I would hope that no one would undertake such a deceitful course with either the Church or their intended spouse at the start of a marriage.