I would always scratch Luna's butt from the time she was a few days old not realizing I was teaching her to back up to me every time she wanted a good butt scratching. I neglected to tell my husband this though, and once she backed up to him so he starts panicking thinking she's getting ready to kick and asks me what he should do. I said, "she just wants her butt scratched."
I think it's pretty harmless, too. Just inform others what to do if she presents her butt, I guess! Plus, you did a great thing by desensitizing what was once "touchy" area.
Stella - sweet, timid, elegant, lovely, lively, amazing
Luna - large, unattractive, naughty, adored