Why do men's pants have a right-hand zipper and women's pants have a left-hand zipper?

This is something that has bothered me for years. I don’t get it. Why not one standard zipper? Why?? It’s sort of awkward, that left-hand zipper, if you’re right-handed and used to wearing men’s pants. Ahem. :)

Also, did you know that when zippers were first invented, conservative religious people protested them, saying it would encourage fornication because it would make entry into the pants easier and faster?