Objection 1. It would seem that rape is not a species of lust, distinct from seduction. For Isidore says (Etym. v, 26) that "seduction [stuprum], or rape, properly speaking, is unlawful intercourse, and takes its name from its causing corruption: wherefore he that is guilty of rape is a seducer." Therefore it seems that rape should not be reckoned a species of lust distinct from seduction.
Objection 2. Further, rape, apparently, implies violence. For it is stated in the Decretals (XXXVI, qu. 1 [Append. Grat. ad can. Lex illa]) that "rape is committed when a maid is taken away by force from her father's house that after being violated she may be taken to wife." But the employment of force is accidental to lust, for this essentially regards the pleasure of intercourse. Therefore it seems that rape should not be reckoned a determinate species of lust.
Objection 3. Further, the sin of lust is curbed by marriage: for it is written (1 Corinthians 7:2): "For fear of fornication, let every man have his own wife." Now rape is an obstacle to subsequent marriage, for it was enacted in the council of Meaux: "We decree that those who are guilty of rape, or of abducting or seducing women, should not have those women in marriage, although they should have subsequently married them with the consent of their parents." Therefore rape is not a determinate species of lust distinct from seduction.
Objection 4. Further, a man may have knowledge of his newly married wife without committing a sin of lust. Yet he may commit rape if he take her away by force from her parents' house, and have carnal knowledge of her. Therefore rape should not be reckoned a determinate species of lust.
On the contrary, Rape is unlawful sexual intercourse, as Isidore states (Etym. v, 26). But this pertains to the sin of lust. Therefore rape is a species of lust.
I answer that, Rape, in the sense in which we speak of it now, is a species of lust: and sometimes it coincides with seduction; sometimes there is rape without seduction, and sometimes seduction without rape.
They coincide when a man employs force in order unlawfully to violate a virgin. This force is employed sometimes both towards the virgin and towards her father; and sometimes towards the father and not to the virgin, for instance if she allows herself to be taken away by force from her father's house. Again, the force employed in rape differs in another way, because sometimes a maid is taken away by force from her parents' house, and is forcibly violated: while sometimes, though taken away by force, she is not forcibly violated, but of her own consent, whether by act of fornication or by the act of marriage: for the conditions of rape remain no matter how force is employed. There is rape without seduction if a man abduct a widow or one who is not a virgin. Hence Pope Symmachus says [Ep. v ad Caesarium; Cf. can. Raptores xxxvi, qu. 2, "We abhor abductors whether of widows or of virgins on account of the heinousness of their crime."
There is seduction without rape when a man, without employing force, violates a virgin unlawfully.
Reply to Objection 1. Since rape frequently coincides with seduction, the one is sometimes used to signify the other.
Reply to Objection 2. The employment of force would seem to arise from the greatness of concupiscence, the result being that a man does not fear to endanger himself by offering violence.
Reply to Objection 3. The rape of a maiden who is promised in marriage is to be judged differently from that of one who is not so promised. For one who is promised in marriage must be restored to her betrothed, who has a right to her in virtue of their betrothal: whereas one that is not promised to another must first of all be restored to her father's care, and then the abductor may lawfully marry her with her parents' consent. Otherwise the marriage is unlawful, since whosoever steals a thing he is bound to restore it. Nevertheless rape does not dissolve a marriage already contracted, although it is an impediment to its being contracted. As to the decree of the council in question, it was made in abhorrence of this crime, and has been abrogated. Wherefore Jerome [The quotation is from Can. Tria. xxxvi, qu. 2 declares the contrary: "Three kinds of lawful marriage," says he, "are mentioned in Holy Writ. The first is that of a chaste maiden given away lawfully in her maidenhood to a man. The second is when a man finds a maiden in the city, and by force has carnal knowledge of her. If the father be willing, the man shall endow her according to the father's estimate, and shall pay the price of her purity [Cf. Deuteronomy 22:23-29. The third is, when the maiden is taken away from such a man, and is given to another at the father's will."
We may also take this decree to refer to those who are promised to others in marriage, especially if the betrothal be expressed by words in the present tense.
Reply to Objection 4. The man who is just married has, in virtue of the betrothal, a certain right in her: wherefore, although he sins by using violence, he is not guilty of the crime of rape. Hence Pope Gelasius says [Can. Lex illa, xxvii, qu. 2; xxxvi, qu. 1]: "This law of bygone rulers stated that rape was committed when a maiden, with regard to whose marriage nothing had so far been decided, was taken away by force."
Source: Summa Theologica (New Advent)