(1) What is considered rude is a personal matter. To me it is invading other people's privacy without an invitation. This is done both in personal life, and also by the tabloid press.

(2) The real purpose of criticizing invasions of privacy is to have a more civilized society. I wouldn't call people who invade privacy "prudes", I'd call them jerks.

It depends what the subject and context is.

To me rude means being uncivil, or failing to treat others with dignity and respect. Prudes are people who have extreme sensibilities as to what is "proper."