Objection 1. It would seem that there is no mortal sin in touches and kisses. For the Apostle says (Ephesians 5:3): "Fornication and all uncleanness, or covetousness, let it not so much as be named among you, as becometh saints," then he adds: "Or obscenity" (which a gloss refers to "kissing and fondling"), "or foolish talking" (as "soft speeches"), "or scurrility" (which "fools call geniality--i.e. jocularity"), and afterwards he continues (Ephesians 5:5): "For know ye this and understand that no fornicator, or unclean, or covetous person (which is the serving of idols), hath inheritance in the kingdom of Christ and of God," thus making no further mention of obscenity, as neither of foolish talking or scurrility. Therefore these are not mortal sins.
Objection 2. Further, fornication is stated to be a mortal sin as being prejudicial to the good of the future child's begetting and upbringing. But these are not affected by kisses and touches or blandishments. Therefore there is no mortal sin in these.
Objection 3. Further, things that are mortal sins in themselves can never be good actions. Yet kisses, touches, and the like can be done sometimes without sin. Therefore they are not mortal sins in themselves.
On the contrary, A lustful look is less than a touch, a caress or a kiss. But according to Matthew 5:28, "Whosoever shall look on a woman to lust after her hath already committed adultery with her in his heart." Much more therefore are lustful kisses and other like things mortal sins. Further, Cyprian says (Ad Pompon, de Virgin., Ep. lxii), "By their very intercourse, their blandishments, their converse, their embraces, those who are associated in a sleep that knows neither honor nor shame, acknowledge their disgrace and crime." Therefore by doing these things a man is guilty of a crime, that is, of mortal sin.
I answer that, A thing is said to be a mortal works. sin in two ways. First, by reason of its species, and in this way a kiss, caress, or touch does not, of its very nature, imply a mortal sin, for it is possible to do such things without lustful pleasure, either as being the custom of one's country, or on account of some obligation or reasonable cause. Secondly, a thing is said to be a mortal sin by reason of its cause: thus he who gives an alms, in order to lead someone into heresy, sins mortally on account of his corrupt intention. Now it has been stated above (I-II, 74, 8), that it is a mortal sin not only to consent to the act, but also to the delectation of a mortal sin. Wherefore since fornication is a mortal sin, and much more so the other kinds of lust, it follows that in such like sins not only consent to the act but also consent to the pleasure is a mortal sin. Consequently, when these kisses and caresses are done for this delectation, it follows that they are mortal sins, and only in this way are they said to be lustful. Therefore in so far as they are lustful, they are mortal sins.
Reply to Objection 1. The Apostle makes no further mention of these three because they are not sinful except as directed to those that he had mentioned before.
Reply to Objection 2. Although kisses and touches do not by their very nature hinder the good of the human offspring, they proceed from lust, which is the source of this hindrance: and on this account they are mortally sinful.
Reply to Objection 3. This argument proves that such things are not mortal sins in their species.
Source: Summa Theologica (New Advent)