Kiddushin 75A

כל שאתה נושא בתו ⁠– אתה נושא אלמנתו. וכל שאי אתה נושא בתו ⁠– אי אתה נושא אלמנתו.

Anyone whose daughter you may marry, you may marry his widow. If a priest is permitted to marry someone’s daughter, he is likewise permitted to marry that person’s widow; she has not become disqualified to marry a priest by having had sex with her husband. And anyone whose daughter you may not marry, you may not marry his widow either.

מאי איכא בין תנא קמא ובין רבי יוסי? אמר רבי יוחנן: מצרי שני איכא בינייהו. ושניהם לא למדוה אלא מכהן גדול באלמנה,

The Talmud asks: What difference is there between the first tanna of the Mishnah and R' Yosei, as they appear to be saying the same thing? R' Yoḥanan says: The practical difference between them concerns a Jewish woman who had sex with a second-generation Egyptian. The Torah prohibits Egyptian converts and their children from entering into the congregation by marriage, but the grandchildren of the Egyptian convert, i.e., the third generation, are permitted to marry Jews with unflawed lineage. And both of them learned their respective opinions only from the halakha of a High Priest who had sex with a widow, although they derived the halakha in different ways.

דתנא קמא סבר: כי כהן גדול באלמנה. מה כהן גדול באלמנה שביאתו בעבירה ופוסל בה, אף כל שביאתו בעבירה ⁠– פוסל.

How so? As the first tanna holds that this case is like that of a High Priest who has sex with a widow: Just as with regard to a High Priest who has sex with a widow, his act of sex with her is performed by means of a transgression and he disqualifies her from marrying into the priesthood, so too anyone whose act of sex is by means of a transgression, such as a second-generation Egyptian who has sex with a Jewish woman, likewise disqualifies her from marrying into the priesthood.