Objection 1. It would seem that there cannot be sin in the sensuality. For sin is proper to man who is praised or blamed for his actions. Now sensuality is common to us and irrational animals. Therefore sin cannot be in the sensuality.
Objection 2. Further, "no man sins in what he cannot avoid," as Augustine states (De Lib. Arb. iii, 18). But man cannot prevent the movement of the sensuality from being inordinate, since "the sensuality ever remains corrupt, so long as we abide in this mortal life; wherefore it is signified by the serpent," as Augustine declares (De Trin. xii, 12,13). Therefore the inordinate movement of the sensuality is not a sin.
Objection 3. Further, that which man himself does not do is not imputed to him as a sin. Now "that alone do we seem to do ourselves, which we do with the deliberation of reason," as the Philosopher says (Ethic. ix, 8). Therefore the movement of the sensuality, which is without the deliberation of reason, is not imputed to a man as a sin.
On the contrary, It is written (Romans 7:19): "The good which I will I do not; but the evil which I will not, that I do": which words Augustine explains (Contra Julian. iii, 26; De Verb. Apost. xii, 2,3), as referring to the evil of concupiscence, which is clearly a movement of the sensuality. Therefore there can be sin in the sensuality.
I answer that, As stated above (2,3), sin may be found in any power whose act can be voluntary and inordinate, wherein consists the nature of sin. Now it is evident that the act of the sensuality, or sensitive appetite, is naturally inclined to be moved by the will. Wherefore it follows that sin can be in the sensuality.
Reply to Objection 1. Although some of the powers of the sensitive part are common to us and irrational animals, nevertheless, in us, they have a certain excellence through being united to the reason; thus we surpass other animals in the sensitive part for as much as we have the powers of cogitation and reminiscence, as stated in the I, 78, 4. In the same way our sensitive appetite surpasses that of other animals by reason of a certain excellence consisting in its natural aptitude to obey the reason; and in this respect it can be the principle of a voluntary action, and, consequently, the subject of sin.
Reply to Objection 2. The continual corruption of the sensuality is to be understood as referring to the "fomes," which is never completely destroyed in this life, since, though the stain of original sin passes, its effect remains. However, this corruption of the "fomes" does not hinder man from using his rational will to check individual inordinate movements, if he be presentient to them, for instance by turning his thoughts to other things. Yet while he is turning his thoughts to something else, an inordinate movement may arise about this also: thus when a man, in order to avoid the movements of concupiscence, turns his thoughts away from carnal pleasures, to the considerations of science, sometimes an unpremeditated movement of vainglory will arise. Consequently, a man cannot avoid all such movements, on account of the aforesaid corruption: but it is enough, for the conditions of a voluntary sin, that he be able to avoid each single one.
Reply to Objection 3. Man does not do perfectly himself what he does without the deliberation of reason, since the principal part of man does nothing therein: wherefore such is not perfectly a human act; and consequently it cannot be a perfect act of virtue or of sin, but is something imperfect of that kind. Therefore such movement of the sensuality as forestalls the reason, is a venial sin, which is something imperfect in the genus of sin.
Source: Summa Theologica (New Advent)