Avodah Zarah 23A

רבינא אמר: לא קשיא ⁠— הא לכתחלה, הא דיעבד.

§ The Talmud cites another resolution of the apparent contradiction between the Mishnah, which rules that non-Jews are suspected of bestiality, and the baraita, which permits an animal purchased from non-Jews to be sacrificed as an offering. Ravina said that it is not difficult; this Mishnah issues its ruling with regard to the halakha ab initio, while that baraita is referring to the halakha after the fact.

ומנא תימרא דשאני בין לכתחלה בין לדיעבד? דתנן: לא תתייחד אשה עמהם, מפני שחשודין על העריות. ורמינהו: האשה שנחבשה בידי גוים, על ידי ממון ⁠— מותרת לבעלה, על ידי נפשות ⁠— אסורה לבעלה!

The Talmud asks: And from where do you say that there is a difference in this case between ab initio and after the fact? As we learned in the Mishnah: A woman may not seclude herself with them because they are suspected of engaging in forbidden sex. And one can raise a contradiction from another Mishnah (Ketubot 26b): With regard to a woman who was imprisoned by non-Jews, if she was imprisoned due to monetary matters she is permitted to her husband even if he is a priest, as there is no concern that she was raped. If she was imprisoned due to a capital offense she is forbidden to her husband if he is a priest, as the captors would not restrain themselves from raping her. The first clause of the Mishnah in Ketubot rules that a woman who was imprisoned in seclusion with non-Jews is not assumed to have had sex with them. This apparently contradicts the statement of the Mishnah here, which rules that a woman may not seclude herself with non-Jews.

אלא לאו שמע מינה, שאני לן בין לכתחלה לדיעבד? ממאי? דלמא לעולם אימא לך: אפילו דיעבד נמי לא, והכא היינו טעמא, דמתיירא משום הפסד ממונו.

The Talmud continues: Rather, isn’t it correct to conclude from here that there is a difference for us between ab initio, as in the Mishnah here, and after the fact, as in the Mishnah in Ketubot? The Talmud rejects this conclusion: From where can this be proven? Perhaps I could actually say to you: Generally, even after the fact, one may not assume that a woman who was secluded with a non-Jew did not have sex with him, and here, in the Mishnah in Ketubot, this is the reason that she is permitted to her husband even after having been imprisoned: Since her husband might not agree to pay if his wife was raped, the non-Jew is fearful of raping her due to the potential loss of his money.