We don't use the word "butt". I don't know why, but we don't. Just never have. We have always said "your behind". You can say it normally, "behind" or with emphasis, "Bee-hind". I don't really know why we use that, but it is what it is.
And honestly, the word "butt" isn't crass, but the sentences it is usually heard in, tend to be more so than if other words are substituted.... so there is some "context" involved in it for me. Generally speaking though, we don't use any of them all that much. We never "kick your butt" or "sit your butt on the sofa" or anything like that. Just isn't the way we communicate in our house, I guess. But those are the types of phraseology (?) I tend to hear "butt" used in most frequently by other people.
When potty training, I told my kids to be sure their behind was clean. Not, "be sure your butt is clean". It just sounds different to me.
And "stupid" has been a not allowed word since the get-go with our kids. Although my kids have heard me call other drivers on the road doing brainless things "idiot" a few times, I don't call them stupid.
I don't think we are snobby, but I do think that words can elevate conversations or lower them. And the more you use certain kinds of words, the less impact they have.