Objection 1. It would seem that fornication is the most grievous of sins. For seemingly a sin is the more grievous according as it proceeds from a greater sensuous pleasure. Now the greatest sensuous pleasure is in fornication, for a gloss on 1 Corinthians 7:9 says that the "flame of sensuous pleasure is most fierce in lust." Therefore it seems that fornication is the gravest of sins.
Objection 2. Further, a sin is the more grievous that is committed against a person more closely united to the sinner: thus he sins more grievously who strikes his father than one who strikes a stranger. Now according to 1 Corinthians 6:18, "He that committeth fornication sinneth against his own body," which is most intimately connected with a man. Therefore it seems that fornication is the most grievous of sins.
Objection 3. Further, the greater a good is, the graver would seem to be the sin committed against it. Now the sin of fornication is seemingly opposed to the good of the whole human race, as appears from what was said in the foregoing Article. It is also against Christ, according to 1 Corinthians 6:15, "Shall I... take the members of Christ, and make them the members of a harlot?" Therefore fornication is the most grievous of sins.
On the contrary, Gregory says (Moral. xxxiii, 12) that the sins of the flesh are less grievous than spiritual sins.
I answer that, The gravity of a sin may be measured in two ways, first with regard to the sin in itself, secondly with regard to some accident. The gravity of a sin is measured with regard to the sin itself, by reason of its species, which is determined according to the good to which that sin is opposed. Now fornication is contrary to the good of the child to be born. Wherefore it is a graver sin, as to its species, than those sins which are contrary to external goods, such as theft and the like; while it is less grievous than those which are directly against God, and sins that are injurious to the life of one already born, such as murder.
Reply to Objection 1. The sensual pleasure that aggravates a sin is that which is in the inclination of the will. But the sensual pleasure that is in the sensitive appetite, lessens sin, because a sin is the less grievous according as it is committed under the impulse of a greater passion. It is in this way that the greatest sensual pleasure is in fornication. Hence Augustine says (De Agone Christiano [Serm. ccxciii; ccl de Temp.; see Appendix to St. Augustine's works]) that of all a Christian's conflicts, the most difficult combats are those of chastity; wherein the fight is a daily one, but victory rare: and Isidore declares (De Summo Bono ii, 39) that "mankind is subjected to the devil by carnal lust more than by anything else," because, to wit, the vehemence of this passion is more difficult to overcome.
Reply to Objection 2. The fornicator is said to sin against his own body, not merely because the pleasure of fornication is consummated in the flesh, which is also the case in gluttony, but also because he acts against the good of his own body by an undue resolution and defilement thereof, and an undue association with another. Nor does it follow from this that fornication is the most grievous sin, because in man reason is of greater value than the body, wherefore if there be a sin more opposed to reason, it will be more grievous.
Reply to Objection 3. The sin of fornication is contrary to the good of the human race, in so far as it is prejudicial to the individual begetting of the one man that may be born. Now one who is already an actual member of the human species attains to the perfection of the species more than one who is a man potentially, and from this point of view murder is a more grievous sin than fornication and every kind of lust, through being more opposed to the good of the human species. Again, a Divine good is greater than the good of the human race: and therefore those sins also that are against God are more grievous. Moreover, fornication is a sin against God, not directly as though the fornicator intended to offend God, but consequently, in the same way as all mortal sins. And just as the members of our body are Christ's members, so too, our spirit is one with Christ, according to 1 Corinthians 6:17, "He who is joined to the Lord is one spirit." Wherefore also spiritual sins are more against Christ than fornication is.
Source: Summa Theologica (New Advent)