How does a word get classified as proper or improper (foul) speech in the first place?
Often, terms that refer to parts of the body or bodily functions that people are uncomfortable discussing in public are classified as improper except in very limited situations and audiences (like doctors, etc).
Insults, however, can be composed of even the most acceptable words.
Technically speaking, “curse” words only describe those expressions designed to put a “hex” on another person and have nothing to do with foul language.