I'm sure there's someone somewhere who's broken it all down bit by bit and figured out reasons why we do some things one way or other things another way, but yeah, I doubt I could explain it properly all here, but I'll have a little stab.
I get that the female body is a normal, natural thing, but then again, so is me jerking off. I don't think I ought to have the right to jerk off though, even in my own home, if I happen to be standing in front of a window facing out to the street.
So yeah, exposing your naked breasts to the street is exposing a sexual part of the body to the public, even though it's obviously nowhere near in the same ball park as jerking off in public, it's sexual enough that society on a whole has deemed it taboo in most places.
Personally, I'd be willing to bet $1000 bucks that the ideas behind things like slut walks were born out of thirsty male brains.