I don’t know about anybody else, but I understand this reaction completely.
I have one sister and two brothers, and like anyone who lives with others for an extended amount of time - accidental walk ins happen all the time.
It does not, however, keep me from wanting to claw my eyes out any less though.
I know how Charles gets a lot of shit for his treatment of Raven, and this particular scene is mentioned quite a bit, but I don’t see this as him rejecting Raven’s mutation.
All I see is an older brother being highly uncomfortable with his little sister’s nudity. Which is, IMO, normal.
THIS. THIS ALL OVER. I get really annoyed whenever I see Charles getting so much stick over this. I know I wouldn’t be comfortable if my brother or sister waltzed into the kitchen naked, blue scales or not. He is not trying to oppress her mutation, he’s awkward because his sister is stood there nekkid.