Actually prior to the Crusades bathing was not common for most medieval people, there was actually those that believed to much washing caused illness and other negative affects. Regular bathing, use of soap and such was something that was largely picked up from the East, along with many other things, from food, architecture, art and so forth...
It seems like a natural thing. And as we are looking at it in a religious, Christian perspective it is hard to see why it should be considered a sin. Sexual urges and desires are encoded into our being, thus are natural. I have to wonder why God would design people to enjoy such feelings if he didn't want them to, if it was only for reproduction. After all, why not just make us like so many other creatures that go into heat or something, so its purely instinctual? Why make us desire such things if we shouldn't?
I think I am getting at masturbation is not a sin. Just as menstruation is not a sin, and desire is not a sin and all those other contradictory things are not sins..