Kommentar zu Wajikra 15:21

ורחץ במים, “he will have to immerse himself in a ritual bath and he remains ritually impure until nightfall.” It goes without saying that his partner in intercourse in a state of menstruation will have to do no less than the male. [The Torah here does not speak of the parties involved having deliberately ignored her state of menstruation, an act which is punished by death through heavenly intervention (Leviticus 20,18 for both.) Here the scenario is that during intercourse the woman suddenly sees some blood not having expected to do so. [No violation of the Torah’s law had been intended by either party. Ed.]