Summa Theologica, by St. Thomas Aquinas, [1947], at sacred-texts.com
We must now consider the object of the consent. Under this head there are two points of inquiry:
(1) Whether the consent that makes a marriage is a consent to carnal intercourse?
(2) Whether consent to marry a person for an immoral motive makes a marriage?
Objection 1: It would seem that the consent which makes a marriage is a consent to carnal intercourse. For Jerome [*The words quoted are found implicitly in St. Augustine (De Bono Viduit ix)] says that "for those who have vowed virginity it is wicked, not only to marry, but even to wish to marry." But it would not be wicked unless it were contrary to virginity, and marriage is not contrary to virginity except by reason of carnal intercourse. Therefore the will's consent in marriage is a consent to carnal intercourse.
Objection 2: Further, whatever there is in marriage between husband and wife is lawful between brother and sister except carnal intercourse. But there cannot lawfully be a consent to marriage between them. Therefore the marriage consent is a consent to carnal intercourse.
Objection 3: Further, if the woman say to the man: "I consent to take thee provided however that you know me not," it is not a marriage consent, because it contains something against the essence of that consent. Yet this would not be the case unless the marriage consent were a consent to carnal intercourse. Therefore, etc.
Objection 4: Further, in everything the beginning corresponds to the consummation. Now marriage is consummated by carnal intercourse. Therefore, since it begins by the consent, it would seem that the consent is to carnal intercourse.
On the contrary, No one that consents to carnal intercourse is a virgin in mind and body. Yet Blessed John the evangelist after consenting to marriage was a virgin both in mind and body. Therefore he did not consent to carnal intercourse.
Further, the effect corresponds to its cause. Now consent is the cause of marriage. Since then carnal intercourse is not essential to marriage, seemingly neither is the consent which causes marriage a consent to carnal intercourse.
I answer that, The consent that makes a marriage is a consent to marriage, because the proper effect of the will is the thing willed. Wherefore, according as carnal intercourse stands in relation to marriage, so far is the consent that causes marriage a consent to carnal intercourse. Now, as stated above (Q[44], A[1]; Q[45], AA[1],2), marriage is not essentially the carnal union itself, but a certain joining together of husband and wife ordained to carnal intercourse, and a further consequent union between husband and wife, in so far as they each receive power over the other in reference to carnal intercourse, which joining together is called the nuptial bond. Hence it is evident that they said well who asserted that to consent to marriage is to consent to carnal intercourse implicitly and not explicitly. For carnal intercourse is not to be understood, except as an effect is implicitly contained in its cause, for the power to have carnal intercourse, which power is the object of the consent, is the cause of carnal intercourse, just as the power to use one's own property is the cause of the use.
Reply to Objection 1: The reason why consent to marriage after taking the vow of virginity is sinful, is because that consent gives a power to do what is unlawful: even so would a man sin if he gave another man the power to receive that which he has in deposit, and not only by actually delivering it to him. With regard to the consent of the Blessed Virgin, we have spoken about it above (Sent. iv, D, 3; TP, Q[29], A[2]).
Reply to Objection 2: Between brother and sister there can be no power of one over the other in relation to carnal intercourse, even as neither can there be lawfully carnal intercourse itself. Consequently the argument does not prove.
Reply to Objection 3: Such an explicit condition is contrary not only to the act but also to the power of carnal intercourse, and therefore it is contrary to marriage.
Reply to Objection 4: Marriage begun corresponds to marriage consummated, as habit or power corresponds to the act which is operation.
The arguments on the contrary side show that consent is not given explicitly to carnal intercourse; and this is true.
Objection 1: It would seem that marriage cannot result from one person's consent to take another for a base motive. For there is but one reason for one thing. Now marriage is one sacrament. Therefore it cannot result from the intention of any other end than that for which it was instituted by God; namely the begetting of children.
Objection 2: Further, the marriage union is from God, according to Mat. 19:6, "What . . . God hath joined together let no man put asunder." But a union that is made for immoral motives is not from God. Therefore it is not a marriage.
Objection 3: Further, in the other sacraments, if the intention of the Church be not observed, the sacrament is invalid. Now the intention of the Church in the sacrament of matrimony is not directed to a base purpose. Therefore, if a marriage be contracted for a base purpose, it will not be a valid marriage.
Objection 4: Further, according to Boethius (De Diff., Topic. ii) "a thing is good if its end be good." But matrimony is always good. Therefore it is not matrimony if it is done for an evil end.
Objection 5: Further, matrimony signifies the union of Christ with the Church; and in this there can be nothing base. Neither therefore can marriage be contracted for a base motive.
On the contrary, He who baptizes another for the sake of gain baptizes validly. Therefore if a man marries a woman for the purpose of gain it is a valid marriage.
Further, the same conclusion is proved by the examples and authorities quoted in the text (Sent. iv, D, 30).
I answer that, The final cause of marriage may be taken as twofold, namely essential and accidental. The essential cause of marriage is the end to which it is by its very nature ordained, and this is always good, namely the begetting of children and the avoiding of fornication. But the accidental final cause thereof is that which the contracting parties intend as the result of marriage. And since that which is intended as the result of marriage is consequent upon marriage, and since that which comes first is not altered by what comes after, but conversely; marriage does not become good or evil by reason of that cause, but the contracting parties to whom this cause is the essential end. And since accidental causes are infinite in number, it follows that there can be an infinite number of such causes in matrimony, some of which are good and some bad.
Reply to Objection 1: This is true of the essential and principal cause; but that which has one essential and principal end may have several secondary essential ends, and an infinite number of accidental ends.
Reply to Objection 2: The joining together can be taken for the relation itself which is marriage, and that is always from God, and is good, whatever be its cause; or for the act of those who are being joined together, and thus it is sometimes evil and is not from God simply. Nor is it unreasonable that an effect be from God, the cause of which is evil, such as a child born of adultery; for it is not from that cause as evil, but as having some good in so far as it is from God, although it is not from God simply.
Reply to Objection 3: The intention of the Church whereby she intends to confer a sacrament is essential to each sacrament, so that if it be not observed, all sacraments are null. But the intention of the Church whereby she intends an advantage resulting from the sacrament belongs to the well-being and not to the essence of a sacrament; wherefore, if it be not observed, the sacrament is none the less valid. Yet he who omits this intention sins; for instance if in baptism one intend not the healing of the mind which the Church intends. In like manner he who intends to marry, although he fail to direct it to the end which the Church intends, nevertheless contracts a valid marriage.
Reply to Objection 4: This evil which is intended is the end not of marriage, but of the contracting parties.
Reply to Objection 5: The union itself, and not the action of those who are united, is the sign of the union of Christ with the Church: wherefore the conclusion does not follow.