Objection 1. It would seem that carnal sins are not of less guilt than spiritual sins. Because adultery is a more grievous sin than theft: for it is written (Proverbs 6:30-32): "The fault is not so great when a man has stolen... but he that is an adulterer, for the folly of his heart shall destroy his own soul." Now theft belongs to covetousness, which is a spiritual sin; while adultery pertains to lust, which is a carnal sin. Therefore carnal sins are of greater guilt than spiritual sins.
Objection 2. Further, Augustine says in his commentary on Leviticus [The quotation is from De Civ. Dei ii, 4 and iv, 31.] that "the devil rejoices chiefly in lust and idolatry." But he rejoices more in the greater sin. Therefore, since lust is a carnal sin, it seems that the carnal sins are of most guilt.
Objection 3. Further, the Philosopher proves (Ethic. vii, 6) that "it is more shameful to be incontinent in lust than in anger." But anger is a spiritual sin, according to Gregory (Moral. xxxi, 17); while lust pertains to carnal sins. Therefore carnal sin is more grievous than spiritual sin.
On the contrary, Gregory says (Moral. xxxiii, 11) that carnal sins are of less guilt, but of more shame than spiritual sins.
I answer that, Spiritual sins are of greater guilt than carnal sins: yet this does not mean that each spiritual sin is of greater guilt than each carnal sin; but that, considering the sole difference between spiritual and carnal, spiritual sins are more grievous than carnal sins, other things being equal. Three reasons may be assigned for this. The first is on the part of the subject: because spiritual sins belong to the spirit, to which it is proper to turn to God, and to turn away from Him; whereas carnal sins are consummated in the carnal pleasure of the appetite, to which it chiefly belongs to turn to goods of the body; so that carnal sin, as such, denotes more a "turning to" something, and for that reason, implies a closer cleaving; whereas spiritual sin denotes more a "turning from" something, whence the notion of guilt arises; and for this reason it involves greater guilt. A second reason may be taken on the part of the person against whom sin is committed: because carnal sin, as such, is against the sinner's own body, which he ought to love less, in the order of charity, than God and his neighbor, against whom he commits spiritual sins, and consequently spiritual sins, as such, are of greater guilt. A third reason may be taken from the motive, since the stronger the impulse to sin, the less grievous the sin, as we shall state further on (6). Now carnal sins have a stronger impulse, viz. our innate concupiscence of the flesh. Therefore spiritual sins, as such, are of greater guilt.
Reply to Objection 1. Adultery belongs not only to the sin of lust, but also to the sin of injustice, and in this respect may be brought under the head of covetousness, as a gloss observes on Ephesians 5:5. "No fornicator, or unclean, or covetous person," etc.; so that adultery is so much more grievous than theft, as a man loves his wife more than his chattels.
Reply to Objection 2. The devil is said to rejoice chiefly in the sin of lust, because it is of the greatest adhesion, and man can with difficulty be withdrawn from it. "For the desire of pleasure is insatiable," as the Philosopher states (Ethic. iii, 12).
Reply to Objection 3. As the Philosopher himself says (Ethic. vii, 6), the reason why it is more shameful to be incontinent in lust than in anger, is that lust partakes less of reason; and in the same sense he says (Ethic. iii, 10) that "sins of intemperance are most worthy of reproach, because they are about those pleasures which are common to us and irrational minds": hence, by these sins man is, so to speak, brutalized; for which same reason Gregory says (Moral. xxxi, 17) that they are more shameful.
Source: Summa Theologica (New Advent)