Did Thomas Aquinas really say that rape was less of a sin than masturbation? His text itself seems pretty darned clear and unambiguous, but commentators’ views range from “he didn’t really mean that”, to “well, it kind of is”. A parable on the danger of multi-step chains of abstract reasoning, and on how easy it is to reinterpret old texts after modernity has produced the right answer.
The actual question explicitly under discussion is whether one sin is less gravely lustful than another; since Aquinas explicitly says that masturbation is one kind of mortal sin but rape is two kinds of mortal sin, one has to ask what the conditions are under which something is "less of a sin" by being doubly sinful. And a little common sense and critical thought shows that the interpretation that has the Catholic moral theologian saying explicitly that masturbation is worse than raping nuns is, far from being "pretty darned clear and unambiguous", quite as dubious as it sounds. There's no actual way to read the structure of the question as being "clear and unambiguous" in the way proposed; Aquinas very clearly specifies that the vices against nature are the gravest "in tali materia", which is a technical term that restricts the topic under discussion, and this technical restriction has been explicitly operative throughout the entire discussion prior to this article, so it's not like it's out of nowhere to anyone who has actually bothered to read the context. This is all quite easily confirmed when we look at the grounds for ranking the second class of sins of lust, the ones proceeding on natural principles: simple fornication is the least because it involves no injustice, adultery is worse because it is also an act of injustice, rape is worse because it is not just an act of lust and injustice but is also violent (violence is not an additional vice but something that makes injustice worse than it would otherwise be -- violent injustices are injustices committed against someone's will), and that each of these can be made even worse by also being an act of sacrilege. And as Bolin notes, injustice is recognized elsewhere as (considered properly in itself) a worse vice than lust. (Also, as Bolin points out in the comments, Aquinas asks this kind of question of a lot of sins, and in every other case his answer is restricted to the particular feature of the act relevant to the particular species of sin under discussion, so why it should suddenly not be the case here would be quite the mystery, and would need to be addressed.)
Thus there's only one way to read the structure of the article. There are two species of lust:
(1) Lust against the principles of nature
(2) Lust consistent with the principles of nature
The first is according to Aquinas the worst species of sexual deformity or deficiency, which is the matter of lust; that is to say, the relevant acts are, when deliberately chosen, most thoroughly expressive of lust as such. Particular acts of each species, however, may by circumstances also become acts of even worse vices, which Aquinas again is very clear about elsewhere (even in other articles of this very question), and within each species the worst acts are those that are also the acts of the worst other vices. It is worth pointing out that the Kainz article also recognizes this. The only issue is that Kainz reads the ordering as strictly linear. But given the specification is in terms of gravity in the matter of the action, the strictly linear ordering can't be maintained; it depends crucially on taking the passage to be concerned with gravity in the form of the action. Also, contrary to what Kainz suggests, sacrilege is stated to be an aggravating circumstance for each of the sexual sins consistent with the principles of nature, and thus the ordering Kainz gives is not quite right even on his own assumptions. Given the fact that there is a sacrilegious version of each sexual sin and a violent version of each sexual sin, the strictly linear interpretation is already in trouble: there's no clear line in this linear order, and sacrilege can't in any case have the place it's given by Kainz. But even Kainz explicitly recognizes that "the extraneous factors of violence and injustice...can magnify the overall sinfulness of the action"; given the way Aquinas characterizes rape, violence and injustice are not strictly extraneous to it, so either this is where Kainz is actually misstepping or he is simply not being clear.
So, in short, no, you can't ignore technical terms explicitly in the text or the broader context which sets the universe of discourse and approach, and what we really learn from this case is how easy it is to misinterpret philosophical texts if you miss or ignore technical distinctions or broader context. Interpretation of an argument is a rational activity accountable to evidence; there is no "clear and unambiguous" except "clear and unambiguous in light of all the relevant evidence".
Since evidence can be overlooked or misclassified, there is no particular problem with reinterpreting a text when a better assessment of the evidence comes in, and this not uncommonly happens through rediscovery (old logical texts, for instance, are constantly having to be interpreted in light of new discoveries, since sometimes the only way to identify a forgotten logical discovery is by rediscovering it independently and discovering that it's a rediscovery); but we see in any case that it's not so clear that "modernity" has "produced the right answer" in the case of rape: any pretense that "modernity" has a particularly good handle on rape ignores the very long list of problems modern conceptions of rape are known to have. The assumption that has to be made in the passage quoted above, for it actually to work given that Aquinas is talking about lust, is that rape is especially evil either because it is really lustful or because it is a really perverse form of lust. But as Aquinas quite explicitly says, rape is especially grave because it is a sexual act of injustice involving the violation of another person, and clearly this is a far superior account of the evil of rape. Rape is undeniably a rationally defective act in sexual matter, which is what makes it an act of lust; but no one in their right mind should conclude that its evil is summed up in its being very unrestrained or inappropriate sex -- or one would hope, although in practice our society, despite some unconvincing lip service to the importance of consent to kinda-sorta get the notion of 'violation' in, does often seem to treat rape as if it were precisely that. I think it's certainly possible to progress beyond Aquinas in the discussion of the subject, but assuming that lust is the only vice relevant to the evil of rape, or that rape's wrongness consists in its being especially lustful, is regress, not progress.
I'm somewhat amused, incidentally, that this is supposed to be a parable about "the danger of multi-step chains of abstract reasoning" given that the argument, even if it could be interpreted in the way suggested, is less than fifteen sentences long and has only a few steps; if there's any problem with it, it shows the danger of not putting in enough steps.
Also incidentally, I remember actually intending to criticize Kainz's interpretation when it first came out, but apparently never got around to it. Apparently this misreading needs to be nipped in the bud, though.