I'm reading Leonard Schlain's Sex, Time, and Power: How Women's Sexuality Shaped Human Evolution, in which Schlain discusses the origin of the species and how gyna sapien (as opposed to homo sapien, Latin for wise man) changed the direction in which our species headed. I'm in the midst of a chapter about menstruation called "Periods/Perils". Allow me to quote something I found very interesting.
(Emphasis mine.)If a wounded animal bleeds excessively, a hunter anticipates that it will soon collapse. Animals that bleed intuit instinctively that they have been injured, and will retreat to the back of the cave or burrow to lick their wounds. A predatory homonid male would be acutely aware that copious bleeding in a wounded animal is an event preceding its death.
Imagine, then, the awe, fright, and confusion that men experienced when the furtively caught sight of a woman's menses. Women bled, but they did not grow weak. They bled, but they were not injured. They bled, but did not die. Sexual relations with a menstruating female would conclude with the male's withdrawing with a blood-smeared member. Feeding many male's innate castration fears, this disturbing sight would tend to cool a man's ardor and make him believe that a menstruating woman possessed a power beyond his ken. Menses would seem to him to be some sort of magic. Perceived supernatural powers induce fear, and men began to fear women. This in turn led men to resent women, because, even though they were bigger and stronger, men were afraid of the otherworldly supremacy they imputed to women.
In other words, ancient man's fear of ancient woman created a hatred of women that has been passed down through the generations and made just as innate as "many male's innate castration fears" because this fear of women (that turned to hatred) began at the dawn of our species when gyna sapien began menstruating.
For the record, guys, women's periods don't imbue us with special powers. In all honestly, no one's really sure what purpose menstruation does serve, but we do know that it doesn't make us magical, and that there are no "menetoxins" or "bad humors" being flushed from our bodies each month with our periods. Gynecologists aren't even that certain any more that women even need to have periods, which is probably why those of us who have decided that we don't want monthly periods (or periods at all) haven't imploded, exploded, or suffered any real health problems for lack of periods.
By the way, Sex, Time, and Power is an awesome book so far. I recommend you check it out. (Actually, I believe I already have recommended it. Twice.)