Objection 1. It would seem that it is not a mortal sin to have intercourse with a concubine. For a lie is a greater sin than simple fornication: and a proof of this is that Juda, who did not abhor to commit fornication with Thamar, recoiled from telling a lie, saying (Genesis 38:23): "Surely she cannot charge us with a lie." But a lie is not always a mortal sin. Neither therefore is simple fornication.
Objection 2. Further, a deadly sin should be punished with death. But the Old Law did not punish with death intercourse with a concubine, save in a certain case (Deuteronomy 22:25). Therefore it is not a deadly sin.
Objection 3. Further, according to Gregory (Moral. xxxiii, 12), the sins of the flesh are less blameworthy than spiritual sins. Now pride and covetousness, which are spiritual sins, are not always mortal sins. Therefore fornication, which is a sin of the flesh, is not always a mortal sin.
Objection 4. Further, where the incentive is greater the sin is less grievous, because he sins more who is overcome by a lighter temptation. But concupiscence is the greatest incentive to lust. Therefore since lustful actions are not always mortal sins, neither is simple fornication a mortal sin.
On the contrary, Nothing but mortal sin excludes from the kingdom of God. But fornicators are excluded from the kingdom of God (1 Corinthians 6:9-10). Therefore simple fornication is a mortal sin.
Further, mortal sins alone are called crimes. Now all fornication is a crime according to Tobit 4:13, "Take heed to keep thyself... from all fornication, and beside thy wife never endure to know crime." Therefore, etc.
I answer that, As we have already stated (Sent. ii, D, 42, 1, 4), those sins are mortal in their genus which violate the bond of friendship between man and God, and between man and man; for such sins are against the two precepts of charity which is the life of the soul. Wherefore since the intercourse of fornication destroys the due relations of the parent with the offspring that is nature's aim in sexual intercourse, there can be no doubt that simple fornication by its very nature is a mortal sin even though there were no written law.
Reply to Objection 1. It often happens that a man who does not avoid a mortal sin, avoids a venial sin to which he has not so great an incentive. Thus, too, Juda avoided a lie while he avoided not fornication. Nevertheless that would have been a pernicious lie, for it would have involved an injury if he had not kept his promise.
Reply to Objection 2. A sin is called deadly, not because it is punished with temporal, but because it is punished with eternal death. Hence also theft, which is a mortal sin, and many other sins are sometimes not punished with temporal death by the law. The same applies to fornication.
Reply to Objection 3. Just as not every movement of pride is a mortal sin, so neither is every movement of lust, because the first movements of lust and the like are venial sins, even sometimes marriage intercourse. Nevertheless some acts of lust are mortal sins, while some movements of pride are venial: since the words quoted from Gregory are to be understood as comparing vices in their genus and not in their particular acts.
Reply to Objection 4. A circumstance is the more effective in aggravating a sin according as it comes nearer to the nature of sin. Hence although fornication is less grave on account of the greatness of its incentive, yet on account of the matter about which it is, it has a greater gravity than immoderate eating, because it is about those things which tighten the bond of human fellowship, as stated above. Hence the argument does not prove.
Source: Summa Theologica (New Advent)