There's also the implication that because they dress "provocatively" and are sluts they deserve to be raped. It's not the fact that people judge others based on appearance that is the main problem, it's the conclusions they draw because of that "judgement".
A direct comparison, for example, would be if the Constable said that people should avoid dressing like nerds so they don't get assaulted or bullied. It's putting the fault on the victim, not the perpetrator.
On a side note, that is exactly what insurance companies (and to a lesser extent the police too) do. If you don't lock your door, then it's your fault for getting your TV stolen. Not that I'm equating rape and burglary, but it is interesting to see that our world is so inconsistent when it comes to this concept.
Wearing miniskirts in public isn't a indicator. A person who is slutty could and probably would wear provocative clothing. So would someone who thinks it looks good. Are we going to define people by their taste in fashion?
You're still thinking from your own viewpoint, which is that someone has to have a lot of sex to be a slut, while I'm saying that the problem is due to a viewpoint mismatch. To people who define sluts as overly promiscuous, it doesn't matter if they're still virgins or are getting it on with 3 guys, a girl and a dog at the same time, if they behave in a certain way they're a slut. (Not saying I subscribe to that viewpoint). To people who hold this definition, a 'slut walk' is rather confusing, since it is essentially saying "Don't think we're overly promiscuous just because we are overly promiscuous'...