Objection 1. It would seem that no consent can be compulsory. For, as stated above (Sent. ii, D, 25 [I-II, 6, 4]) the free-will cannot be compelled. Now consent is an act of the free-will. Therefore it cannot be compulsory.
Objection 2. Further, violent is the same as compulsory. Now, according to the Philosopher (Ethic. iii, 1), "a violent action is one the principle of which is without, the patient concurring not at all." But the principle of consent is always within. Therefore no consent can be compulsory.
Objection 3. Further, every sin is perfected by consent. But that which perfects a sin cannot be compulsory, for, according to Augustine (De Lib. Arb. iii, 18), "no one sins in what he cannot avoid." Since then violence is defined by jurists (i, ff. de eo quod vi metusve) as the "force of a stronger being that cannot be repulsed," it would seem that consent cannot be compulsory or violent.
Objection 4. Further, power is opposed to liberty. But compulsion is allied to power, as appears from a definition of Tully's in which he says that "compulsion is the force of one who exercises his power to detain a thing outside its proper bounds." Therefore the free-will cannot be compelled, and consequently neither can consent which is an act thereof.
On the contrary, That which cannot be, cannot be an impediment. But compulsory consent is an impediment to matrimony, as stated in the text (Sent. iv, D, 29). Therefore consent can be compelled.
Further, in marriage there is a contract. Now the will can be compelled in the matter of contracts; for which reason the law adjudges that restitution should be made of the whole, for it does not ratify "that which was done under compulsion or fear" (Sent. iv, D[29). Therefore in marriage also it is possible for the consent to be compulsory.
I answer that, Compulsion or violence is twofold. One is the cause of absolute necessity, and violence of this kind the Philosopher calls (Ethic. iii, 1) "violent simply," as when by bodily strength one forces a person to move; the other causes conditional necessity, and the Philosopher calls this a "mixed violence," as when a person throws his merchandise overboard in order to save himself. In the latter kind of violence, although the thing done is not voluntary in itself, yet taking into consideration the circumstances of place and time it is voluntary. And since actions are about particulars, it follows that it is voluntary simply, and involuntary in a certain respect (Cf. I-II, 6, 6). Wherefore this latter violence or compulsion is consistent with consent, but not the former. And since this compulsion results from one's fear of a threatening danger, it follows that this violence coincides with fear which, in a manner, compels the will, whereas the former violence has to do with bodily actions. Moreover, since the law considers not merely internal actions, but rather external actions, consequently it takes violence to mean absolute compulsion, for which reason it draws a distinction between violence and fear. Here, however, it is a question of internal consent which cannot be influenced by compulsion or violence as distinct from fear. Therefore as to the question at issue compulsion and fear are the same. Now, according to lawyers fear is "the agitation of the mind occasioned by danger imminent or future" (Ethic. iii, 1).
This suffices for the Replies to the Objections; for the first set of arguments consider the first kind of compulsion, and the second set of arguments consider the second.
Source: Summa Theologica (New Advent)