" nevertheless I would be thoroughly justified in accusing them of filthy manners."

As Del G say: if all we're talking about is you expressing your disapproval of someone's clothes or manners, fine. Personally, I don't think I would ever find myself on a position where I thought that a stranger's clothes were so offensive that they would ever provoke me to confront them, but in itself that's not a crime. But saying that a girl wearing the t-shirt you describe "deserved" some sort of punishment, spanking or otherwise, makes me uncomfortable.