Midrash for Leviticus 15:18

7) (Vayikra 15:18) ("And a woman, if a man lie with her, a lying of semen, then they shall bathe in water, and they shall be unclean until the evening.") "a woman": to exclude a minor. I might then think to exclude a child of three years and one day; it is, therefore, written "and a woman."

8) "if a man lie with her": to exclude a minor (less than nine years old). "her": to exclude a bride, (a woman not conceiving from her first coitus). These are the words of R. Yehudah. They said to him: What does this (verse) have to do with a bride? Rather, "her," to exclude unnatural coitus (i.e., sodomy). ("a lying of) semen": to exclude peripheral contact (through which she does not conceive).

9) "then they shall bathe in water, and they shall be unclean until the evening": Her bathing is being likened to his bathing. Just as his bathing must be in cleanliness (i.e., without anything intervening between the water and his flesh), so her bathing must be in cleanliness. Just as his bathing is in forty sa'ah, so her bathing must be in forty sa'ah. Just as his bathing excludes hidden parts, so her bathing excludes hidden parts.