Temurah 30A

רבי אלעזר היא, דאמר: פנוי הבא על הפנויה שלא לשום אישות עשאה זונה. אי רבי אלעזר, מאי איריא אלמנה? ניתני פנויה!

It is in accordance with the opinion of R' Elazar, who says: With regard to an unmarried man who had sex with an unmarried woman not for the purpose of marriage, he has thereby caused her to become a prostitute. It is for this reason that her payment is prohibited to be sacrificed on the altar. By contrast, Abaye holds in accordance with the opinion of the Rabbis, who disagree with R' Elazar. The Talmud asks: If the baraita is in accordance with the opinion of R' Elazar, why mention specifically the prohibition of a widow who is paid for having sex with a High Priest? Let it teach that payment is prohibited even in the case of an unmarried man who had sex with an unmarried woman.

אלמנה איצטריך, סלקא דעתך אמינא: הואיל ובנין אב הוא ⁠— לא ניתסרו, קא משמע לן.

The Talmud answers that it was necessary for the tanna to cite the example of a widow who is paid for having sex with a High Priest, as it might enter your mind to say: Since the case of an unmarried man who had sex with an unmarried woman, which is not a forbidden relationship by Torah law, is the paradigm example of prohibited payment to a prostitute, perhaps in a situation where the relationship is forbidden by Torah law, the payment should not be prohibited. Therefore, the baraita teaches us that payment to a prostitute is prohibited to be sacrificed on the altar even when the relationship is forbidden by Torah law, as in the case of a widow who is paid for having sex with a High Priest.