Pro-tip to young trans guys:
If a stranger misgenders you, please please please do not ever utter the phrase, “I’m a man.” It sounds very unnatural and immediately sounds overly defensive.
My advice? Just look at the person like they’re an idiot and, in the deepest voice possible, say, “Uh. Alright, then.”
Just act as though they made a huge and obvious mistake, and don’t get flustered. If you’re comfortable with it, handle the situation with humor and say something like, “Man, I know I’ve got a babyface, but I didn’t think it was that bad.”
People are saying that you should be unapologetic but the keyword here is “stranger”. You could be in danger if the person is transphobic, and you have no way of knowing their stance if you don’t know the person. So writing it off casually ensures the most low risk way of making your gender known.
Posting this again because of the new information added on.