Though Aquinas clearly affirms that the influence of passions on the will can decrease the degree of voluntariness, and thus the merit or demerit of good actions or sins, when it comes to gravely disordered acts, such as fornication, masturbation, or theft, he still sees the issue as basically black or white: either the act is voluntary, and then it is a mortal sin; or it is not voluntary, and then it is not properly a sin at all. He affirms this position both in the De Malo, and in the Summa Theologiae.
De Malo, question 3, article 10
Whether sins committed out of passion are imputed to man as mortal fault
It should be said that since men sometimes commit adultery out of weakness or passion, and do many crimes or shameful things, as Peter did when he denied Christ out of fear, no one should have any doubt that sins perpetrated out of passion are sometimes mortal.
To understand this we should consider that the necessity based on something one's will has power over does not keep an act from being a mortal sin, just as, if someone stabs a dagger in someone's vital members it is necessary that that man die, but stabbing him is voluntary. Hence the death of the man who is stabbed is imputed to the one who struck him as a mortal fault.
And it is similar in the case we are considering; for given that reason is bound by passion, it is necessary that a disordered choice follow, but the will has the power to repel this bond of reason. For it was said that reason is bound due to the soul's intention being vehemently drawn to the act of the sensitive appetite; hence it is turned away from considering in particular that which it knows universally. But the will has the power to turn the intention to something or not. Hence it has the power to keep reason from being bound. Therefore the act committed on account of reason's being bound is voluntary; hence it is not excused from fault, even mortal fault. But if the bond of reason by passion goes so far that it is not in the will's power to remove such a bond, as if someone becomes insane through some passion of the soul, whatever he commits would not be imputed to him as fault, as neither would it be imputed to any insane person—unless perhaps such a passion was voluntary from its very beginning; for then the will could from the beginning prevent the passion for going so far; thus murder committed through drunkenness is imputed to man as fault, since the beginning of drunkenness was voluntary.
Summa Theologiae I-II, Question 77, Article 8
Whether a sin committed out of passion can be mortal
It should be said that mortal sin, as was said above, consists in turning away from the last end, which is God; this turning away pertains to the reason in its deliberation, since it is reason that orders things to the end. Therefore the only way in which the soul's inclination to something which is contrary to the last end can be not a mortal sin, is because reason is unable to resist this inclination by its deliberation, as is the case in sudden movements of the passions. But when someone out of passion goes on to do a sinful act, or to deliberate consent, this does not happen suddenly. Hence reason by deliberating can resist this further act; for it can get rid of, or at least prevent it [from leading to action], as was said above. Hence if it does not resist it, it is a mortal sin. Thus we see that many murders and adulteries are committed out of passion.
In response to the third objection, which argues that mortal sin consists in turning away from God, and only man's spiritual faculty of reason can do this, while passion cannot strictly speaking turn toward or away from God, Aquinas argues:
It should be said that passion does not always impede reason totally from its act; hence it retains free judgment, so that it can turn away from or toward God. If, however, the use of reason were totally taken away, there would be neither a mortal nor a venial sin.
This response of Aquinas seems to indicate that his position is in fact quite a bit stronger than that "sin committed out of passion can be mortal"–that in his mind, every act that is objectively gravely disordered, if it is at all voluntary, though committed under the influence of passion, is a mortal sin.