When speaking about the influence of passions on the will, Aquinas takes the position that so long as people retain the use of reason and free will, if they are moved by passion to do a gravely disordered act, then they sin mortally. Only if they are so overcome by passion that they no longer have the ability to judge and to act freely are they excused from mortal sin, as they are excused from sin altogether.
St. Thomas takes a similar hard-line position regarding ignorance of universal principles of law, such as the prohibition of fornication, at least in his later writings. (In De Malo q. 3, a. 8 Aquinas says that if an act is done in ignorance in one respect, and knowingly in another, then it is voluntary in the respect it which it is done knowingly, and involuntary in the respect in which it is done in ignorance, as when someone does not know that fornication is a sin, he voluntarily commits fornication, but does not voluntarily commit a sin–and Aquinas does not add anything about the fornication being indirectly voluntary, because of neglect in acquiring knowledge about its sinfulness.)
But in the Summa Theologiae and in the later part of the De Malo Aquinas considers the case of a person who believes that fornication is a venial sin, and is of such a mindset that he would definitely refrain from fornication, if he knew it were a mortal sin. He puts forth the argument in an objection: the difference between mortal and venial sin is that the person who sins venially loves some creature more than he ought, yet loves it less than God, while the person who sins mortally loves some creature (at least himself or his own will) more than he loves God. But a person with such a mindset seems obviously to love the good he is seeking less than he loves God, since he would be willing to forego it if he knew it was contrary to the love of God. In the Summa he responds to this by saying that if the ignorance entirely excuses from sin, the person would of course not commit a mortal sin, since they would not sin at all. But if the ignorance is not invincible, and does not entirely excuse from sin, then the ignorance itself is a sin, and contains in it a lack of divine love, inasmuch as a man neglects to learn those things through which he can preserve himself in divine love. (ST I-II 88:6 ad 2; see also ST III 80:4 ad 5; De Malo q. 7, a. 1, obj 18 and response).
Similarly in a Quodlibetal question he says, “sometimes an erroneous conscience does not absolve or excuse from sin, namely when the error itself is a sin, proceeding from ignorance of that which someone is able to and obliged to know, as for example, if someone believed fornication to be simply a venial sin, and then, [if he committed fornication], although he would believe that he was sinning venially, he would not be sinning venially, but mortally” (Quodlibetal 8, q. 6, a. 5)
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