Objection 1. It would seem that compulsory consent does not invalidate a marriage. For just as consent is necessary for matrimony, so is intention necessary for Baptism. Now one who is compelled by fear to receive Baptism, receives the sacrament. Therefore one who is compelled by fear to consent is bound by his marriage.
Objection 2. Further, according to the Philosopher (Ethic. iii, 1), that which is done on account of mixed violence is more voluntary than involuntary. Now consent cannot be compelled except by mixed violence. Therefore it is not entirely involuntary, and consequently the marriage is valid.
Objection 3. Further, seemingly he who has consented to marriage under compulsion ought to be counseled to stand to that marriage; because to promise and not to fulfill has an "appearance of evil," and the Apostle wishes us to refrain from all such things (1 Thessalonians 5:22). But that would not be the case if compulsory consent invalidated a marriage altogether. Therefore, etc.
On the contrary, A Decretal says (cap. Cum locum, De sponsal. et matrim.): "Since there is no room for consent where fear or compulsion enters in, it follows that where a person's consent is required, every pretext for compulsion must be set aside." Now mutual contract is necessary in marriage. Therefore, etc.
Further, Matrimony signifies the union of Christ with the Church, which union is according to the liberty of love. Therefore it cannot be the result of compulsory consent.
I answer that, The marriage bond is everlasting. Hence whatever is inconsistent with its perpetuity invalidates marriage. Now the fear which compels a constant man deprives the contract of its perpetuity, since its complete rescission can be demanded. Wherefore this compulsion by fear which influences a constant man, invalidates marriage, but not the other compulsion. Now a constant man is reckoned a virtuous man who, according to the Philosopher (Ethic. iii, 4), is a measure in all human actions.
However, some say that if there be consent although compulsory, the marriage is valid in conscience and in God's sight, but not in the eyes of the Church, who presumes that there was no inward consent on account of the fear. But this is of no account, because the Church should not presume a person to sin until it be proved; and he sinned if he said that he consented whereas he did not consent. Wherefore the Church presumes that he did consent, but judges this compulsory consent to be insufficient for a valid marriage.
Reply to Objection 1. The intention is not the efficient cause of the sacrament in baptism, it is merely the cause that elicits the action of the agent; whereas the consent is the efficient cause in matrimony. Hence the comparison fails.
Reply to Objection 2. Not any kind of voluntariness suffices for marriage: it must be completely voluntary, because it has to be perpetual; and consequently it is invalidated by violence of a mixed nature.
Reply to Objection 3. He ought not always to be advised to stand to that marriage, but only when evil results are feared from its dissolution. Nor does he sin if he does otherwise, because there is no appearance of evil in not fulfilling a promise that one has made unwillingly.
Source: Summa Theologica (New Advent)