Full context:   St. Thomas Aquinas differentiates between sexual intercourse within marriage, which is seen as meritorious through giving justice to one's spouse, and sins of lust which can themselves be differentiated in magnitude of immorality according to intention and action. For example, Aquinas says in the Summa Theologica II-II, q. 154, a. 12 "I answer that, In every genus, worst of all is the corruption of the principle on which the rest depend. 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." He uses St. Augustine as his source writing "Augustine says that 'of all these,' namely the sins belonging to lust, 'that which is against nature is the worst.'" Which St. Thomas clarifies means that they are greater than sins against justice pertaining to the genus of lust, such as rape or incest, in his statement "Reply to Objection 3: The nature of the species is more intimately united to each individual, than any other individual is. Wherefore sins against the specific nature are more grievous." Thus St. Thomas gives the order of magnitude of lustful acts as: "The most grievous is the sin of bestiality, because use of the due species is not observed...(Then) the sin of sodomy, because use of the right sex is not observed...(Then) the sin of not observing the right manner of copulation (or the unatural act or masturbation)... (Then) incest... is contrary to the natural respect which we owe persons related to us... Then, it is a greater injustice to have intercourse with a woman who is subject to another's authority as regards the act of generation, than as regards merely her guardianship. Wherefore adultery is more grievous than seduction. And both of these are aggravated by the use of violence."