I'm currently reading The World Until Yesterday by Jared Diamond. It's about traditional hunter-gatherer societies, the way in which humanity lived for most of its history. The book discusses the handful of still-extant hunter-gatherer peoples as a way of looking back in time to the way our ancestors would have lived. In a section discussing childbirth, Diamond mentions that among the Kaulong people of New Britain (an island off of New Guinea) the men are afraid of contamination from a woman during menstruation and childbirth. That's awfully familiar sounding. Intrigued, I did a little poking around online.
The idea that menstruating women are impure and contact with them is harmful is widespread, both in the ancient world and in modern-day primitive cultures. Pliny the Elder wrote in 77 AD, “Contact with menstrual blood turns new wine sour, crops touched by it become barren, grafts die…" Traditional societies have various taboos surround menstruation. "Among the Beng of Ivory Coast, a menstruating woman may not touch a corpse, cook for old men or set foot in the forest. She may not touch the logs or coals on the fire of a non-menstruating woman." " According to the Eskimo, if a man comes into physical contact with a menstruating woman, an invisible vapor will attach itself to the man and make him less successful at hunting." " In Malekula, an island of the New Hebrides, a menstruating woman may not enter a garden in which young plants are growing."
Menstruation taboos have also held on in some of the world's major religions. A Muslim woman who is menstruating is forbidden from performing certain religious activities or having sex, and Hindu women are similarly prohibited from certain activities and must be purified (just as Judaism requires women to tovel in a mikvah) before resuming those activities. Judaism, of course, has niddah.
One of the most common features of the menstruation taboos is a requirement for the woman to remove herself from society for the duration of her period. In many of these societies, there are special communal huts for women who are menstruating. Ancient Judaism, too , required a niddah to go chutz lamachaneh.
One theory as to why menstruation has been seen as dangerous and polluting is that it is something outside of the way things normally are. Things that are unusual - not rare, necessarily, but not the usual way things are - are often seen as dangerous and polluting. Dead bodies, for instance, are not rare, but are different from the usual live people we interact with, and so have been seen as dangerous and polluting (e.g, tammei). Blood usually comes from a cut. Blood without an obvious source is therefore unusual, and is dangerous and polluting. In a patriarchal society, as traditional societies usually are, the holy man who declares which things are impure is, well, a man, for whom bleeding without an obvious source is doubly unusual. Add to that the fact that, unlike now, in the ancient world women rarely got their periods (because of inadequate nutrition, frequent pregnancies, and the suppression of menstruation by frequent nursing) and that no one understood why women bled. Even for women, the bleeding was an unusual, suspect event.
Niddah, despite modern whitewashing, is NOT about improving marriages or making sex more special. It's about fear of contamination from the miasma of impurity around a menstruating woman, a fear that is not particular to Judaism, ordained from on high by an all-knowing God, but that is common to primitive cultures around the world. It is a relic of the primitive period of the culture that would go on to evolve into the magnificent culture of the Jewish people, preserved as a religious dictate that could not be abandoned.
While in the distant past niddah and the accompanying separation between husband and wife would have been a rare occurrence, something that happened once every few years, today it has become an onerous burden, imposing itself on couples for half of every month. Frum couples today have to endure niddah as many times in a year as couples in the past would experience in their lifetime.
The one thing I wonder is if the particular practices associated with the menstruation taboo was originated by men or women. Was it men who, fearful of their wives' impurity and the general ickiness of a week of constant bleeding decided to banish women to menstruation huts and forbid sex with them, or was it women who, feeling lousy during their periods, invented menstruation huts as a way to get away from domestic chores and banned sex so that their husbands wouldn't bother them while they weren't feeling well?