While multitudes would love to adapt to culture, and/ or use "culture" as an excuse to sin,
Yahuweh and His Word does not, and is not dependent on cultural changes, and does not change with man's changes, nor can anyone rely on nor trust "culture".
Agreed. However, you're also misunderstanding what I was saying.
The Law was intended to set apart the Jewish people from the surrounding peoples, and furthermore to prepare them to be the ethnic group from which the Messiah would come. That doesn't make the Law contrary to God's will, but we are no longer under that covenant (Galatians 3:15-25). Insisting that we must still be under Levitical Law goes directly against Scripture (Galatians 5:4-5).
The moral aspects of the Law remain the same—murder is still wrong, rape is still wrong, etc.—but we also have freedom. We shouldn't misuse that freedom (Galatians 5:13-14), but we are not under the law in the same manner which the Israelites were.
Furthermore, the Law has facets which apply more directly to the situation the people of Israel were in: How should they run their society? What about these superstitions and rituals from these other cultures around them? Is it okay to act like them?
Granted, I am no biblical scholar, but lots of these instances exist to further set apart the Jewish people from the surrounding area, whether that is in regards to the dynamics of their marriages or to their treatment of animals.
My speculation earlier specifically involved the use of menstrual blood in magic. The Jewish people of the time would probably have had some questions about this. That would have been a legitimate issue for Jewish people of the time. It would have been very relevant for them.
Another possibility is protection against disease. An issue of blood between periods can be a sign of disease (anything from blood clotting disorders to STDs). Maybe there was concern about how to diagnose if a woman was having an irregular period or if something was medically wrong. In an era with very little knowledge about the diagnosis and treatment of disease, a standard by which to determine whether a woman was healthy (clean) or not would have been vital. (It would also make sense to include such a passage where the diagnosis of possible STDs in men is discussed as well).
Again, I'm just spitballing over here, but even so, my speculation isn't meant to say this passage doesn't matter today. It's a reminder that God cares even about the minutiae of our lives. In a culture where women are often told not to talk about aspects of our health (including our menstrual health) because it's uncomfortable or gross, it's comforting to know that it's not a subject God is uncomfortable talking about at all. So it's still very relevant on that front.