It's because they've seen so many shapeshifter characters in movies, the Incredible Hulk, etc. that they think you can completely change your body like that in the real world.

I laugh. And laugh. And laugh, every time I see sexual dimorphism hit these guys smack in the face. And they get mad about it!!

This is a good ROFLMAO:
"I sent off for a 23andMe DNA profile and it said that I'm XY. I'm so hurt by that. Can they re-do the test"

Lol that reminds me of a post (probably on reddit, but I don't remember) where a woman was saying her tim boyfriend needed an Ancestry DNA test that would ignore his Y chromosome.