I live in a weird mix of English and French (Montreal) :) It's just surprising to me since butt always was less formal than "posterior" and less rude than what

I live in a weird mix of English and French (Montreal) :) It's just surprising to me since butt always was less formal than "posterior" and less rude than what we'd more frankly call an "ass". I definitely want to acknowledge that I know not all environments are similar to mine :) I'm just surprised to hear there are contexts where someone might find the phrase "I've fallen on my butt" to be more rude than if any other body part was mentioned. I would have assumed the problem is less the word choice and the fact that people are being rather prude about bodies and their function, much like grade schoolers are like to be.