A Rashi from last week
Years ago, when I was leyning this parsha in an egalitarian community, I got to the section describing the preparations for the theophany, and when I reached the verse "For three days do not approach near a woman" one of the women in the congregation hissed. In retrospect, I wished that I had had the gumption to go back and read the pasuk again, but now that I have learned the Rashi on that pasuk, I realize that I, and she, and many others have been reading that verse backwards.
This injunction cannot have anything to do with protecting the men from ritual impurity.
First of all, the whole point is that the entire nation should be present for the theophany; we learn midrashically that the blind and deaf miraculously had their sight and hearing restored. Surely God would have been able to adjust the women's menses to ensure that none of them would be niddah on that day.
And even without such a "special arrangement", if a man touches a woman who is niddah, either he'd go to mikvah and be a tevul yom for that single day; or if they have relations, he's tamei for seven days. Three days is either overkill or woefully insufficient!
So what can these three days be guarding against?
A man's seed is also a source of tumah. A man who becomes a baal keri washes in nine kavs of water and ceases to be tamei, but the man's seed is presumed to potentially leak from the woman for --- you guessed it, three days!
So this verse, which to our ears sounds like it's warning the men to avoid becoming tamei from their wives, is actually warning the men not to get their wives tamei!