>>675122
It's Summa, the driest and most coherent theological manual outhere.
If anything it's read like physics textbook.
But since education is dead in 90% of places then let me rephrase it.
<Wherever there occurs a special kind of deformity whereby the venereal act is rendered unbecoming, there is a determinate species of lust.
When sexual act is not rightful (i.e. between man and his wife, with penis in vagina) it's mean that there is special kind of lust.
<This may occur in two ways: First, through being contrary to right reason, and this is common to all lustful vices; secondly, because, in addition, it is contrary to the natural order of the venereal act as becoming to the human race: and this is called "the unnatural vice."
All lustful sins are unreasonable (and thus evil). But some are not only evil in this way but they are also opposite to the very nature of sexual act.
<This may happen in several ways. First, by procuring pollution, without any copulation, for the sake of venereal pleasure: this pertains to the sin of "uncleanness" which some call "effeminacy."
Masturabation is such sin against nature, because it's done without another person for the sake of pleasure. Masturabation is called "uncleanness" which some call "effeminacy."
<In every genus, worst of all is the corruption of the principle on which the rest depend.
The worst kind of corruption is such that is opposite to the very foundation of a thing corrupted.
<Now the principles of reason are those things that are according to nature, because reason presupposes things as determined by nature, before disposing of other things according as it is fitting.
Sins are always against reason. And foundation of reason is nature of thing that reason meditates and acts upon by will.
<This may be observed both in speculative and in practical matters. Wherefore just as in speculative matters the most grievous and shameful error is that which is about things the knowledge of which is naturally bestowed on man
This truth (worst corruption is that of roots) is true to both thoughts and actions. In thoughts, worst error is such that attacks natural knowledge of man.
<so in matters of action it is most grave and shameful to act against things as determined by nature. Therefore, since by the unnatural vices man transgresses that which has been determined by nature with regard to the use of venereal actions, it follows that in this matter this sin is gravest of all.
And when it comes to actions the worst sin is that attacks the very nature of thing. And thus sins against nature, that attacks the very nature of sexual act, such as masturbation, are worst kind of sin.
tl;dr Masturabtion is always unnatural and unnatural sins are worst kind of sins when it comes to lustful ones.