Tell her you grew out of the phase stage yesterday!:lol::lol: But that would suck to be told that, you are right!
''Don't worry mum, soon you'll be in the acceptance phase and I'll talk to you more about me being X when you're ready'' How is that for an answer?
I would argue her and ask her why she thinks it's a phase. Then proceed from there. Eventually leading me to argue that one does not choose what they are attracted to. Works wonders. It's easy to win an argument when you're right.