From the Supplement, compiled after Aquinas’s death from his Commentary on the Sentences and transcribed here from the 1917 to 1922 printings. About the Supplement.
Text: Summa Theologiae, Supplement to the Third Part (Supplementum Tertiae Partis), transcribed from Internet Archive scans, translated by the Fathers of the English Dominican Province. The Supplement, compiled after Aquinas’s death from his earlier Commentary on the Sentences, in the same English Dominican translation as the Gutenberg Parts. Transcribed by Bibliotheca Catholica from the OCR of four Internet Archive scans of the pre-1931 printings (Questions 1 to 33 from the 1917 Washbourne and Benziger volume, 34 to 68 from the 1922 volume, 69 to 86 from the 1921 volume, and 87 to 99 from the 1922 Benziger volume), with OCR errors corrected and doubtful passages marked, not rewritten. 99 questions and 446 articles. The two appendices printed after Question 99 are not included. Sources, SHA-256 of each OCR file, and the method are in the source record. Rights: Public domain (US)
The Supplement is not a Project Gutenberg text. It is Bibliotheca Catholica’s transcription of public-domain scans at the Internet Archive; obvious OCR errors were corrected, and passages marked “OCR uncertain” could not be confidently corrected, so check them against the linked scan page.
Objectio?i I. It would seem that affinity is not an impediment to marriage. For nothing is an impediment to marriage except what is contrary thereto. But affinity is not contrary to marriage since it is caused by it. Therefore it is not an impediment to marriage.
Obj. 2: Further, By marriage the wife becomes a possession of the husband. Now the husband's kindred inherit his possessions after his death. Therefore they can succeed to his wife, although she is affine to them, as shown above (A. 5). Therefore affinity is not an impediment to marriage.
On the contrary, It is written (Lev. xviii. 8): Thou shalt not uncover the nakedness of thy father s wife. Now she is only affine. Therefore affinity is an impediment to marriage.
I answer that, Affinity that precedes marriage hinders marriage being contracted and voids the contract, for the same reason as consanguinity. For just as there is a certain need for blood-relations to live together, so is there for those who are connected by affinity: and just as there is a tie of friendship between blood-relations, so is there between those who are affine to one another. If, however, affinity supervene to matrimony, it cannot void the marriage, as stated above (Q. L., A. 7).
Reply Obj. 1: Affinity is not contrary to the marriage which causes it, but to a marriage being contracted with an affine, in so far as the latter would hinder the extension of friendship and the curbing of concupiscence, which are sought in marriage.
Reply Obj. 2: The husband's possessions do not become one with him as the wife is made one flesh with him. Wherefore just as consanguinity is an impediment to marriage or union with the husband according to the flesh, so is one forbidden to marry the husband's wife.
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Also online, linked only: the same translation at New Advent and the Latin text at Corpus Thomisticum. Neither is copied here.