Birth control is unheard of. You have a litter of kids demanding attention. Also, your husband isn’t very knowledgable about science.
So, he might demand sex.
What would you prefer? Going out there and facing all that. Or, being by yourself in a separate quarter. Where you can rest, be away from infection and not be troubled.
That is what ancient India prescribed. A scientific principle, which ( unfortunately) needed the seal of religion.
Women on their menses were considered charged, according to tantric principles. ( hormonal changes? )
When any woman reaches such a state, it is difficult for Her to perform yogic tasks ( even the simple task of praying).
When Goddess Kamakhya undergoes Her yearly menstruation, Her doors are closed. Maa is left to Herself, to rest and recuperate.
Red strips of clothes are distributed as a part of the blessings and the Devi’s menstrual flow is celebrated.
Religion created rules to make science palatable. Over the years, chauvinists and political activists twisted them to their own will and fantasy.
Understand before you rebel.