Rashi on Leviticus 15:18/רש"י על ויקרא ט״ו:י״ח

1ורחצו 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).