Costumes for boys and for girls

How would you have reacted in the same situation?

You can't be a princess

I can understand the mother reaction, she can think his son doesn't know what he's doing, that he's very young and that's a children crazy idea.

She must be protecting his child: she knows in school he will be the centre of jokes and mockerys.

I think my reaction will be something like this, but I will try to talk with my son and understand why he wants to be a princess.
'You Can't Be a Princess': WWYD?