Sforno on Leviticus 15:33/ספורנו על ויקרא ט״ו:ל״ג

1והדוה בנדתה, the word דוה alludes to something sinful, hence she became ritually defiled.

2והזב, the thoughts we must entertain when studying this subject is that we must reflect on the original sin.

3את זובו לזכר ולנקבה, at this point we must consider the difference between the legislation governing a man’s involuntary seminal-like ejaculation, and that of a woman. A woman’s discharge confers impurity only if it is reddish, whereas the parallel discharge by a male confers impurity only if it is not reddish.

4ולאיש אשר ישכב עם טמאה, here we also have to reflect on the subject of a man deliberately indulging in sexual intercourse with a ritually contaminated woman. The Torah had written (verse 24) ותהי נדתה עליו, “her state of menstrual separation will apply to him.” There is no parallel verse describing such a result as due to a ritually pure woman sleeping with a man who was a zav at the time. [the ritual impurity of the zav was induced by a diseased body, as opposed to menstruation which is a natural state ever since the original sin. Ed.] There appears to be a dichotomy there. The Torah teaches that when a man is violating sexual mores he causes more spiritually negative fallout to himself than when a woman does the same thing in reverse. At least, there appears to be a difference between sickness induced by individual sin, such as zav and zavah as well as in the case of tzoraat, which we explained as also a sickness due to individual sin, not to the spiritual state of man, the species, since the original sin. This is why voluntary carnal association with the menstruant woman is a greater sin than voluntary carnal association with zav or zavah.