Remember, this is the easy one, to your mother, you are and always will be her little boy and he had hair, and second--this is very much harder to accept and agree to, men by in large marry their "mothers", it's Freud don't fight the concept--and to the wife you are also "their little boy" hence hair if that's the way they met you.
You can't fight it with logic, you couldn't when you actually were a little boy, and you can't now. Accept it, don't ask permission, ask forgiveness. If they won't give it, do it anyway, it's something you do for yourself, not them.