You can be what you want in your dreams, except a woman

I've been reading and thinking about gender and transgender a lot, lately (I'm sure I don't have to tell you that Skepchick has launched a sister-site called Queerika. What's a straight cis woman like me doing there? Well, shutting up and listening. Learning. I feel privileged to be able to do so.)
And so it came that a little remark made by a friend lately got me all going.
My kids love playing make-believe. We spend half the days as characters from their favourite books/ movies or animals. So, lately we were playing Winnie-the-Pooh and my oldest daughter handed out the roles. She was Tigger, her sister was Roo, I was Rabbit and then she told her dad that he was Mummy Heffalump.
To which she was chastized by a well-meaning, but absolutely clueless friend that "oh, but your dad can't be Mummy Heffalump, he's a boy, not a girl!".
Thankfully, my husband glossed over the awkward moment by saying that he'd love to be Mummy Heffalump, because he loves Heffalumps and because she's so strong .
BUt there were two things about our friend's remark thst struck me:
First, she never said a thing when the three women in the room turned into male characters*.
Secondly, it was absolutely possible that the dad got turned into a purple, giant, elephant-like creature that can tear up whole trees. It was not possible that he got turned into a woman.
Because that's truely unheard of.

*Most series don't even come with three female characters we could use