Bit of a weird one. This happened in lockdown but I've wondered for a while if I was being unreasonable or not!
Basically my step son who stays with us 50/50, saw me painting my nails and asked me to do his. I didn't see any reason why not to, DH was in the same room, raised an eyebrow slightly and did a chuckle but wasn't bothered. As he wasn't in school, didn't think it was a problem.
He picked a colour out and I did on hand (at his request!) And told him to let me know when he wanted it taking off. He was so chuffed with it, kept showing it to DH and my mum who came round later on etc... 
Anyway, this happened twice. I took it off and he asked me for another colour so we redid it.
Anyway, DH got a bit of a pissed off message from SS's mum basically saying she doesn't want him to come home wearing nail varnish again and why were we even encouraging him to wear it etc etc...
DH basically ignored and said SS had asked and it was just a bit of fun seen as he isn't in school, didn't see a problem and left it there.
Was I unreasonable?! I don't even want to paint my nails around him now in case he asks. I'd feel mean saying no (but would take it off again before he left or went to school).