Yevamot 103B

רעה היא אצל צדיקים. שנאמר: ״השמר לך פן תדבר עם יעקב מטוב עד רע״, בשלמא רע ⁠— לחיי, אלא טוב אמאי לא? אלא שמע מינה: טובתן של רשעים ⁠— רעה היא אצל צדיקים.

is a disadvantage for the righteous, as a righteous individual gains no pleasure from this so-called beneficial act. As it is stated by God to Laban: “Take heed to yourself that you speak not to Jacob either good or bad” (Genesis 31:24). Granted, speak no bad; this is rightly so, i.e., understandable. But speak no good? Why not? Rather, learn from here that even something that would be a good benefit to the wicked like Laban, is a disadvantage for the righteous.

בשלמא התם, דלמא מדכר ליה שמא דעבודה זרה. אלא הכא, מאי רעה איכא?

The Talmud asks: Granted, there, in Laban’s words to Jacob, it is understandable that there could be a certain repulsive aspect to a wicked man speaking nicely to a righteous individual, as perhaps he will mention to him the name of the idol he worships and even though he means well, it still would repulse Jacob. But here, what disadvantage is there if she derives benefit from licentious sex with a wicked man?

דקא שדי בה זוהמא. דאמר רבי יוחנן: בשעה שבא נחש על חוה הטיל בה זוהמא. ישראל, שעמדו על הר סיני ⁠— פסקה זוהמתן. גוים, שלא עמדו בהר סיני ⁠— לא פסקה זוהמתן.

The Talmud answers: He implants filth in her and contaminates her, as her body accepts his semen. As R' Yoḥanan also said, based on his understanding that the serpent seduced Eve into having sex with him: When the serpent came upon Eve, he infected her with moral contamination, and this contamination remained in all human beings. When the Jewish people stood at Mount Sinai their contamination ceased, whereas with regard to non-Jews, who did not stand at Mount Sinai, their contamination never ceased. Therefore, Yael was repulsed by the contamination that she allowed into her body, and she did not benefit from sex with Sisera.