Yoma 6B

אלא לאו, לטבילתן!

Rather, must it not be that the baraita is equating them with regard to their immersion, in that both one who has sex with a menstruating woman and one impure with impurity imparted by a corpse immerse during the day?

לא, לעולם לטומאתן, סיפא אצטריכא ליה: אלא שחמור ממנו בועל נדה, שמטמא משכב ומושב בטומאה קלה, לטמא אוכלין ומשקין.

The Talmud rejects that conclusion. No; actually, the baraita is referring to the duration of their impurity. Although there is nothing novel in that equation, it was necessary for the tanna to teach the latter clause of the baraita: However, in one sense, the case of one who is impure because he had sex with a menstruating woman is more severe than one impure with impurity imparted by a corpse. One who is impure because he had sex with a menstruating woman transmits impurity through lying on a bed or sitting on a chair, even if he never came into direct contact with the chair or the bed. He renders the bed or chair impure with a mild form of impurity. He confers upon them 1st-degree ritual impurity status, sufficient only to render foods and liquids impure. One impure with impurity imparted by a corpse transmits impurity by means of direct contact.

תא שמע, דתני רבי חייא: הזב והזבה והמצורע והמצורעת ובועל נדה וטמא מת טבילתן ביום, נדה ויולדת טבילתן בלילה! תיובתא.

Come and hear an additional proof that R' Ḥiyya taught: For the zav and the zava and the metzora and the female metzora, and one who had sex with a menstruating woman, and one who is ritually impure with impurity imparted by a corpse, their immersion is during the day. For a menstruating woman and a woman after childbirth, their immersion is at night. The Talmud concludes that this baraita is indeed a conclusive refutation of the opinion of Rav Shimi from Neharde’a, who said that one who is impure because he had sex with a menstruating woman immerses at night.