"Really?? Does your mom know?"
"So, how does...that surgery work? Will it look just like a 'real one'?"
"Gah! Why would you get your boobs cut off?! That'd hurt!!"
"Why do you 'want to be a boy'??"
Or, I'll show them another FTM that's had top surgery, etc, and they say "So 'that' (or 'they') are/was a girl?"
It goes on, but it always stems to how the surgery is gonna work. The improper pronouns and 'want to be a boy' thing get to me most. I AM a young man.