Or HaChaim on Leviticus 15:18/אור החיים על ויקרא ט״ו:י״ח

1 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.

2Had it not been for the interpretation of the sages in Torat Kohanim, I would have interpreted the words אשר תצא ממנו שכבת זרע in verse 16 to mean that unless the semen actually leaves the body of the man who became aroused by it, and is visible on the male's flesh, there is no need for a ritual bath because all of it has been absorbed internally. The same law would not apply to a woman; she could become ritually defiled even if the semen entered her without a trace of it being visible on her body. If she touched herself internally and came into contact with the semen she would be ritually impure. The Torah had to write ורחצו, "they have to bathe themselves," to make it clear that both the male and the female partner in such intercourse have to undergo the same procedure to become ritually clean again although the manner in which they each contracted the impurity differed. We are now able to explain the additional letter ו at the beginning of the word ואשה to mean that on occasion also the woman may be subject to the same law as the male in that her hidden parts do not become defiled if her partner cohabited with her in a manner which is not the standard way of conducting sexual intercourse [anal intercourse, for instance, Ed.]. This would be so according to the view of the majority of rabbis cited in Yevamot 34. According to the view of Rabbi Yehudah there, the example of such a הלכה would be a virgin bride on her wedding night. The rabbis derived this rule from the word אותה. Had the Torah not written the word ורחצו and I would have had to figure out the need for both the man and the woman who was his partner to bathe themselves in a ritual bath based only on the additional letter ו in the word ואשה, I would have concluded that the man transmits ritual impurity even if his semen had not seen the light of day. I would have interpreted the word תצא in verse 16 to mean that the semen had left its customary place, i.e. the scrotum, and that it had merely entered the seminal canal of the male member without being ejaculated. Alternatively, I would have understood the words כי תצא as a future tense, i.e. that the man's semen was capable of being ejaculated even though it had not actually been ejaculated, a situation which is similar to the normal situation in a woman. Both would be considered as ritually impure in such a situation. The Torah wrote the word ורחצו to prevent us from pursuing this kind of exegesis, i.e. that in this instance the inference we normally draw from the conjunctive letter ו at the beginning of a paragraph such as the word ואשה does not apply. If the Torah had written אשה אשר ישכב איש אותה ורחצו, i.e. the letter ו at the beginning of the verse would have been absent, I would still have been left with the restrictive meaning mentioned in connection with the word אותה (see above: Rabbi Yehudah, etc.), but I would have to apply this restriction differently having been told explicitly by the text that the woman transmits impurity even if the semen is inside her private parts. In such circumstances there would be no logical reason to differentiate between a virgin-bride (whose hymen was still intact and who therefore could not keep the semen inside her private parts) and any other woman. I would have been forced to conclude that the expression אותה is inclusive rather than restrictive and that the woman transmits impurity if the semen is in any of her private parts (such as her armpits). By adding the letter ו at the beginning of the word אשה, the Torah makes it clear that just as a woman's private parts other than her vagina do not transmit ritual impurity due to the presence of semen, so her vagina does not either. Torat Kohanim made it clear by using the letter inclusively, that the word אותה is to be used only as a restrictive clause. As a result, the letter ו in ואשה was superfluous and could be used to include a girl of three years plus one day and up.