My Latest Theory

Women know this and I'm sure a fair amount of men do too. When you live with other women or are around the same ones all the time, your cycles align. (Like my friend, Mark, said when I had two female roommates, "Eeeesshhhh, don't want to be in that house every 28 days or so.")

I have a theory that this happened because otherwise what would lesbians do? You could potentially be out of service for two weeks a month in a couple like that.