Objection 1. It would seem that seduction should not be reckoned a species of lust. For seduction denotes the unlawful violation of a virgin, according to the Decretals (XXXVI, qu. 1) [Append. Grat. ad can. Lex illa]. But this may occur between an unmarried man and an unmarried woman, which pertains to fornication. Therefore seduction should not be reckoned a species of lust, distinct from fornication.
Objection 2. Further, Ambrose says (De Patriarch. [De Abraham i, 4): "Let no man be deluded by human laws: all seduction is adultery." Now a species is not contained under another that is differentiated in opposition to it. Therefore since adultery is a species of lust, it seems that seduction should not be reckoned a species of lust.
Objection 3. Further, to do a person an injury would seem to pertain to injustice rather than to lust. Now the seducer does an injury to another, namely the violated maiden's father, who "can take the injury as personal to himself" [Gratian, ad can. Lex illa], and sue the seducer for damages. Therefore seduction should not be reckoned a species of lust.
On the contrary, Seduction consists properly in the venereal act whereby a virgin is violated. Therefore, since lust is properly about venereal actions, it would seem that seduction is a species of lust.
I answer that, When the matter of a vice has a special deformity, we must reckon it to be a determinate species of that vice. Now lust is a sin concerned with venereal matter, as stated above (Question 153, Article 1). And a special deformity attaches to the violation of a virgin who is under her father's care: both on the part of the maid, who through being violated without any previous compact of marriage is both hindered from contracting a lawful marriage and is put on the road to a wanton life from which she was withheld lest she should lose the seal of virginity: and on the part of the father, who is her guardian, according to Sirach 42:11, "Keep a sure watch over a shameless daughter, lest at any time she make thee become a laughing-stock to thy enemies." Therefore it is evident that seduction which denotes the unlawful violation of a virgin, while still under the guardianship of her parents, is a determinate species of lust.
Reply to Objection 1. Although a virgin is free from the bond of marriage, she is not free from her father's power. Moreover, the seal of virginity is a special obstacle to the intercourse of fornication, in that it should be removed by marriage only. Hence seduction is not simple fornication, since the latter is intercourse with harlots, women, namely, who are no longer virgins, as a gloss observes on 2 Corinthians 12, "And have not done penance for the uncleanness and fornication," etc.
Reply to Objection 2. Ambrose here takes seduction in another sense, as applicable in a general way to any sin of lust. Wherefore seduction, in the words quoted, signifies the intercourse between a married man and any woman other than his wife. This is clear from his adding: "Nor is it lawful for the husband to do what the wife may not." On this sense, too, we are to understand the words of Numbers 5:13: "If [Vulgate: 'But'] the adultery is secret, and cannot be provided by witnesses, because she was not found in adultery [stupro]."
Reply to Objection 3. Nothing prevents a sin from having a greater deformity through being united to another sin. Now the sin of lust obtains a greater deformity from the sin of injustice, because the concupiscence would seem to be more inordinate, seeing that it refrains not from the pleasurable object so that it may avoid an injustice. On fact a twofold injustice attaches to it. One is on the part of the virgin, who, though not violated by force, is nevertheless seduced, and thus the seducer is bound to compensation. Hence it is written (Exodus 22:16-17): "If a man seduce a virgin not yet espoused, and lie with her, he shall endow her and have her to wife. If the maid's father will not give her to him, he shall give money according to the dowry, which virgins are wont to receive." The other injury is done to the maid's father: wherefore the seducer is bound by the Law to a penalty in his regard. For it is written (Deuteronomy 22:28-29): "If a man find a damsel that is a virgin, who is not espoused, and taking her, lie with her, and the matter come to judgment: he that lay with her shall give to the father of the maid fifty sicles of silver, and shall have her to wife, and because he hath humbled her, he may not put her away all the days of his life": and this, lest he should prove to have married her in mockery, as Augustine observes. [QQ. in Dt., qu. xxxiv.]
Source: Summa Theologica (New Advent)