פירוש על ויקרא 15:18

ורחצו במים THEY SHALL BOTH LAVE THEMSELVES IN WATER — It is a decree of the King that a woman should become unclean through sexual intercourse. The reason is not because of the law of “one who touches שכבת זרע” (who, according to Leviticus 22:7 is unclean; cf. Sifra on that verse) for in her case it is מגע בית הסתרים and this does not render her unclean (Niddah 41b).

ואשה אשר ישכב איש אותה note that the Torah does not write איש כי ישכב את אשה, “when a man sleeps with a woman,” which is the Torah’s normal way of introducing such subjects, i.e. emphasizing the predominating role of the male in the sexual union. The reason for this change in syntax is to teach that unless the colour of the fluid originating in her vagina is reddish it does not confer ritual impurity. The only time the whitish seminal excretion of a woman confers ritual impurity on her male partner is when the man initiates the customary sexual union with her even if the seminal fluid remains in בית הסתרים “hidden” parts within the orifices of the woman, has not seen the light of day.

ורחצו במים, they shall both bathe themselves in a ritual bath. The reason the Torah again mentions the male partner and the requirement that he has to bathe himself -something mentioned expressly in verse 16- is explained by Torat Kohanim as teaching that both he and she are subject to exactly the same procedure when it comes to the immersion in a ritual bath. You may argue that if that was all the verse teaches us there was no need to write ורחצו..וטמאו, but we could have derived all this from the letter ו in the word ואשה, seeing we have numerous instances where that letter was used to compare the laws in a later paragraph to those spelled out in the Torah in a previous paragraph (compare the first few chapters of Leviticus)! The fact is that Torat Kohanim already used the letter ו in the word ואשה to teach that it includes a minor of three years and one day. Accordingly, if the Torah had not written the word ורחצו, I would have interpreted that letter as referring to details about the immersion in the ritual bath instead of interpreting it as including a girl of three years plus. We would have had no extraneous letter or word to teach us that even a three-year old girl would have to purify herself if she had become the victim of sexual intercourse with a male.