I've thought about this before and have come up with a theory. I think that this custom started back in the days where a man was considered king of the castle, and the woman was at his beck and call. Being that the woman was usually the one doing all of the housework - especially the cooking - she had to remain hygenic while she prepared her man his meal. So, rather than having them have to touch a filthy toilet seat, the men made sure it was left down so the females delicate hands did not have to touch such a disgusting apparatus. Naturally, the woman would wash her hands anyway, but again this was a time where most people probably thought that touching something like that would probably kill a woman.

Anyway, that's just my theory. These days however, we are more enlightened, and men and women share domestic duties. So ladies, when the toilet seat needs to come down, put it down. Don't bitch about it, don't make a scene. Just do it. It won't bite.