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 (44, 1; 45, 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; III, 29, 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.
Source: Summa Theologica (New Advent)