Objection 1. It would seem that sin is not alleviated on account of passion. For increase of cause adds to the effect: thus if a hot thing causes something to melt, a hotter will do so yet more. Now passion is a cause of sin, as stated (5). Therefore the more intense the passion, the greater the sin. Therefore passion does not diminish sin, but increases it.
Objection 2. Further, a good passion stands in the same relation to merit, as an evil passion does to sin. Now a good passion increases merit: for a man seems to merit the more, according as he is moved by a greater pity to help a poor man. Therefore an evil passion also increases rather than diminishes a sin.
Objection 3. Further, a man seems to sin the more grievously, according as he sins with a more intense will. But the passion that impels the will makes it tend with greater intensity to the sinful act. Therefore passion aggravates a sin.
On the contrary, The passion of concupiscence is called a temptation of the flesh. But the greater the temptation that overcomes a man, the less grievous his sin, as Augustine states (De Civ. Dei iv, 12).
I answer that, Sin consists essentially in an act of the free will, which is a faculty of the will and reason; while passion is a movement of the sensitive appetite. Now the sensitive appetite can be related to the free-will, antecedently and consequently: antecedently, according as a passion of the sensitive appetite draws or inclines the reason or will, as stated above (1,2; 10, 3); and consequently, in so far as the movements of the higher powers redound on to the lower, since it is not possible for the will to be moved to anything intensely, without a passion being aroused in the sensitive appetite.
Accordingly if we take passion as preceding the sinful act, it must needs diminish the sin: because the act is a sin in so far as it is voluntary, and under our control. Now a thing is said to be under our control, through the reason and will: and therefore the more the reason and will do anything of their own accord, and not through the impulse of a passion, the more is it voluntary and under our control. In this respect passion diminishes sin, in so far as it diminishes its voluntariness.
On the other hand, a consequent passion does not diminish a sin, but increases it; or rather it is a sign of its gravity, in so far, to wit, as it shows the intensity of the will towards the sinful act; and so it is true that the greater the pleasure or the concupiscence with which anyone sins, the greater the sin.
Reply to Objection 1. Passion is the cause of sin on the part of that to which the sinner turns. But the gravity of a sin is measured on the part of that from which he turns, which results accidentally from his turning to something else--accidentally, i.e. beside his intention. Now an effect is increased by the increase, not of its accidental cause, but of its direct cause.
Reply to Objection 2. A good passion consequent to the judgment of reason increases merit; but if it precede, so that a man is moved to do well, rather by his passion than by the judgment of his reason, such a passion diminishes the goodness and praiseworthiness of his action.
Reply to Objection 3. Although the movement of the will incited by the passion is more intense, yet it is not so much the will's own movement, as if it were moved to sin by the reason alone.
Source: Summa Theologica (New Advent)