Ketubot 40A

להא מילתא נמי ליגמרו מהדדי! אמר קרא: ״מהר ימהרנה לו לאשה״, ״לו״⁠ — מדעתו.

for this matter too, marrying the woman against his will, let them be derived from each other. Rav Ashi replied that the verse says with regard to a seducer: “He should pay a dowry for her to be a wife to him” (Exodus 22:15); “to him” means in accordance with his will.

כיצד שותה בעציצו כו׳. אמר רב כהנא: אמריתה לשמעתא קמיה דרב זביד מנהרדעא: ניתי עשה ונדחה לא תעשה!

§ The Mishnah continues: How does the rapist drink from his vessel? The Mishnah proceeds to explain that he is obligated to marry her despite the physical flaws she might have. However, if the marriage is prohibited, either due to the fact that she committed adultery or due to her flawed lineage, he is not obligated to marry her, and therefore he may not marry her. Rav Kahana said: I stated this halakha before Rav Zevid of Neharde’a, and I asked him: Let the positive mitzva: “And to him she should be as a wife” (Deuteronomy 22:29), come and override the prohibition of marriage to a woman who is forbidden to him, according to the principle that positive mitzvot override prohibitions.

אמר לי: היכא אמרינן ניתי עשה ונידחי לא תעשה ⁠— כגון מילה בצרעת, דלא אפשר לקיומיה לעשה. אבל הכא, אי אמרה דלא בעינא, מי איתיה לעשה כלל?

He said to me: Where do we say the principle that a positive mitzva comes and overrides a prohibition? It is in a case where one performs circumcision of a foreskin afflicted with tzara'at. Although there is a prohibition against removing leprous skin, the positive mitzva of circumcision overrides that prohibition, as it is not possible to fulfill the positive mitzva without violating the prohibition. However, here, in the case of the rapist, if she says: I do not want him as a husband, is there a positive mitzva at all? Since in that case the mitzva need not be performed, as it is negated when the woman refuses to marry him, it does not override the prohibition.