Yevamot 98A

הא איסורא איכא! הוא הדין דאפילו איסורא נמי ליכא, ואיידי דבעי למיתני סיפא ״אבל חייבין״, תנא נמי רישא ״אין חייבין״.

there is a rabbinic prohibition, contrary to Rav Aḥa’s opinion. The Talmud answers: The same is true that there is no prohibition, either. And since the baraita wanted to teach in the latter clause that if they were born in sanctity they are liable, it also taught in the first clause that they are not liable. For this reason, the baraita mentions only the absence of liability.

אמר רבא, הא דאמור רבנן: אין אב לגוי, לא תימא משום דשטופי בזמה, דלא ידיע, אבל ידיע ⁠— חיישינן, אלא אפילו דידיע ⁠— נמי לא חיישינן.

Rava said: With regard to that which the rabbis said, that a non-Jew has no patrilineage, do not say that it is because they are so steeped in licentiousness that they do not know the identity of their fathers with certainty, but if that identity is known, we are concerned that the paternity is recognized, with regard to the prohibition of sex with forbidden paternal relatives and other halakhic issues. Rather, even when it is known, we are still not concerned.

דהא שני אחין תאומים, דטפה אחת היא ונחלקה לשתים, וקתני סיפא: לא חולצין ולא מייבמין, שמע מינה: אפקורי אפקריה רחמנא לזרעיה, דכתיב: ״בשר חמורים בשרם וזרמת סוסים זרמתם״.

The proof is from the case of two identical twin brothers, who were one drop that was divided into two and obviously have the same father, and yet it is taught in the latter clause of the baraita: They do not perform ḥalitza and they do not perform levirate marriage, although they certainly have the same father. Learn from this that God dispossesses the male non-Jew of his offspring, as it is written with regard to Egyptians: “Whose flesh is the flesh of donkeys, and whose semen is the semen of horses” (Ezekiel 23:20), i.e., the offspring of a male non-Jew is considered no more related to him than the offspring of donkeys and horses.