I totally understand it... Its because they look so innocent... most of the time the image of the skirt flaring up giving a nice view is all that needs to be explained to understand the obsession

That would be a great explanation, but I don't think that's it at all. Like I said, I can't even begin to explain it, and those are too obvious to not have occurred to me before. I'm sure some of it helps, but guys (well, I, at least) can get the same reaction from a knee-length uniform skirt. I think the librarian obsession is closely related, too.