My mom was the same way at first. Same with my dad. They thought it was just a phase because I'd never acted feminine, but realistically, I'd been trying my whole life to fit in with the other guys, though I still tended to be an outcast. I'm practically certain that everyone here has tried to hide their gender identity, because gender norms are so drilled into children - by parents, by school, by classmates and peers and media and, well, everywhere! So, I really think that argument is one of the most paradoxical.
"Hey, you're a boy, so you need to act like a boy."
"Oh, you want to be a girl? Well, aside from those few times you acted like a girl, which we punished and ridiculed you for, you've always acted like a boy, so obviously you aren't a girl."
"So if you're not a girl, then you're a boy, so you need to act like a boy!"
Stupid circular logic.