in clear words.
tell her that it makes you uncomfortable, that you would prefer if she stopped doing that and that you will decide your own sexuality and what it entails for yourself, without anyone else trying to help.
it’s your sexuality, it’s your label, it’s your “life style choice” if she wants to think about it that way. but it’s yours and she doesn’t get a say in it.
try to be kinda diplomatic, but be clear. she needs to know that she needs to back off.
also, stay calm, take the high road, be the adult one. it’s very satisfying.