I knew one Christian that thought it had to be followed, and I told him pretty much what you told me.
What I don’t understand is what this even MEANS in the Old Testament. (i.e. Why would God ask this of man?)
Well, like many of the levitical laws, it was probably a way for the Israelites to live a life with less infections and diseases than their neighbors. This was likely one of those cases.
Leviticus 15 covers most of the laws regarding uncleanness due to discharge. The additional law in Lev 20 was meant to drive home how serious it is for the time of uncleanness after menstruation to be observed. By having sex during menstruation, both partners are risking the spread of disease that would not only harm them, but can easily spread throughout the community. It is potentially deadly behavior, therefore the punishment is steep.
disease is the best answer I can give you.
I really don’t know but I will look it up