what do you mean by deliberate? isn’t all kissing deliberate?
Things may be deliberate in two ways (sorry, I’ve just been reading Aquinas

): either by the nature of the consent to them or by the manner in which they happen. Hah hah, seriously, few people kiss mechanically, so I obviously don’t mean this kind of deliberate. I mean deliberate as thought, calculated, in a way,
institutionalised. I would worry about premeditated and calculated kissing, which may easily become “something to do”, which verges on foreplay and has the potential to cross the line. If kissing is
institutionalised, as in making out, say people make out daily at a specific hour because that is their custom, it hints of a certain sexual role of that act. When kissing starts to satiate our appetites and we grow dependent on it, that’s very close to lust, perhaps sometimes already there.
As for Aquinas, let’s grab a translation:
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 (FS, Q[74], A[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.
(Emphasis added)
The sensitive part in Latin goes:
Dictum est autem supra quod consensus in delectationem peccati mortalis est peccatum mortale, et non solum consensus in actum. Et ideo, cum fornicatio sit peccatum mortale, et multo magis aliae luxuriae species, consequens est quod consensus in delectationem talis peccati sit peccatum mortale, et non solum consensus in actum. Et ideo, cum oscula et amplexus et huiusmodi propter delectationem huiusmodi fiant, consequens est quod sint peccata mortalia. Et sic solum dicuntur libidinosa. Unde huiusmodi, secundum quod libidinosa sunt, sunt peccata mortalia.
A crude literal translation is this:
*It is said above that the consent to the delectation of a mortal sin is a mortal sin, and not solely the consent to an act. And so, as fornication is a mortal sin, and much more other species of luxury **
, it consequent that consent to the delectation of such a sin is a mortal sin, and not solely the consent to an act. And so, when kisses and caresses and such are done for such delectation, it is consequent that they are mortal sins. And only in this way they are lustful. Hence, this way, because they are lustful, they are mortal sins.
From this follows that kissing for the delectation of mortal sin is a mortal sin. Aquinas points out in the beginning that kissing is *not *a mortal sin of itself, that the “mortality” is determined by the cause. If the cause is lust, then kissing is a mortal sin. This can be done in two ways: either by express intent or by delectation of that lust when it cames.
This doesn’t mean that if you enjoy kissing, it’s mortal for you. It means if you, say, get an erection and accept the carnal feeling of it, then it’s sinful and it has the capacity to be mortal.
Further, a bit more on the philosophical angle, if kissing itself is not a mortal sin but only by a mortal cause, it means kissing is morally neutral of itself (this follows clearly from Aquinas). Delectation in a morally neutral thing cannot be mortal of itself, either. Therefore, delectation in kissing, in so far as no lust is not involved, is not mortally sinful. Not even sinful at all, to be precise. The way we do it, the reasons and causes, inappropriate emphasis, addictions, that’s what makes it sinful.
In practice, one ought to be careful because it’s rather hard to kiss a person you’re specifically attracted to, without provoking a sexual response - sexual at least in a broad sense. I can’t clearly say if any slightest degree of sexual stimulation must be absent from that kissing, or if you are fine if you don’t accept it, i.e. don’t start delighting in it. Using modern biology, we could probably prove that many of the bodily reactions which used to be believed to be non-sexual, are in fact a result of male to female interaction. Heightened pulse would be a prime example and I remember reading a conservative 19th century piece on the morality of kissing and such for teenagers and it specifically said that the thrill, the heightened pulse and such were fine.
In the end, we don’t get a perfectly precise answer from Aquinas, but if we did, we would start lawyering around it and find it imprecise in a number of ways as well in the end. So, it occurs to me we shouldn’t look for the letter but rather for the spirit, which obviously doesn’t mean we can ignore such “letter” as we are aware of (e.g. that kissing to get that feeling down there, pardon my French, is not okay).*