Objection 1. It would seem that the gravity of a sin does not depend on its cause. Because the greater a sin's cause, the more forcibly it moves to sin, and so the more difficult is it to resist. But sin is lessened by the fact that it is difficult to resist; for it denotes weakness in the sinner, if he cannot easily resist sin; and a sin that is due to weakness is deemed less grievous. Therefore sin does not derive its gravity from its cause.
Objection 2. Further, concupiscence is a general cause of sin; wherefore a gloss on Romans 7:7, "For I had not known concupiscence," says: "The law is good, since by forbidding concupiscence, it forbids all evils." Now the greater the concupiscence by which man is overcome, the less grievous his sin. Therefore the gravity of a sin is diminished by the greatness of its cause.
Objection 3. Further, as rectitude of the reason is the cause of a virtuous act, so defect in the reason seems to be the cause of sin. Now the greater the defect in the reason, the less grievous the sin: so much so that he who lacks the use of reason, is altogether excused from sin, and he who sins through ignorance, sins less grievously. Therefore the gravity of a sin is not increased by the greatness of its cause.
On the contrary, If the cause be increased, the effect is increased. Therefore the greater the cause of sin, the more grievous the sin.
I answer that, In the genus of sin, as in every other genus, two causes may be observed. The first is the direct and proper cause of sin, and is the will to sin: for it is compared to the sinful act, as a tree to its fruit, as a gloss observes on Matthew 7:18, "A good tree cannot bring forth evil fruit": and the greater this cause is, the more grievous will the sin be, since the greater the will to sin, the more grievously does man sin.
The other causes of sin are extrinsic and remote, as it were, being those whereby the will is inclined to sin. Among these causes we must make a distinction; for some of them induce the will to sin in accord with the very nature of the will: such is the end, which is the proper object of the will; and by a such like cause sin is made more grievous, because a man sins more grievously if his will is induced to sin by the intention of a more evil end. Other causes incline the will to sin, against the nature and order of the will, whose natural inclination is to be moved freely of itself in accord with the judgment of reason. Wherefore those causes which weaken the judgment of reason (e.g. ignorance), or which weaken the free movement of the will, (e.g. weakness, violence, fear, or the like), diminish the gravity of sin, even as they diminish its voluntariness; and so much so, that if the act be altogether involuntary, it is no longer sinful.
Reply to Objection 1. This argument considers the extrinsic moving cause, which diminishes voluntariness. The increase of such a cause diminishes the sin, as stated.
Reply to Objection 2. If concupiscence be understood to include the movement of the will, then, where there is greater concupiscence, there is a greater sin. But if by concupiscence we understand a passion, which is a movement of the concupiscible power, then a greater concupiscence, forestalling the judgment of reason and the movement of the will, diminishes the sin, because the man who sins, being stimulated by a greater concupiscence, falls through a more grievous temptation, wherefore he is less to be blamed. On the other hand, if concupiscence be taken in this sense follows the judgment of reason, and the movement of the will, then the greater concupiscence, the graver the sin: because sometimes the movement of concupiscence is redoubled by the will tending unrestrainedly to its object.
Reply to Objection 3. This argument considers the cause which renders the act involuntary, and such a cause diminishes the gravity of sin, as stated.
Source: Summa Theologica (New Advent)