There's nothing wrong with saying "you aren't a woman". There's nothing wrong with saying "no, you shouldn't sterilize yourself because you're 13 and confused", and it's not 'being a dick' to say so.
It's being honest. It's showing love. It's proving that you're willing to tell this person the truth, when everyone around them and everyone who claims to love them, is lying to them. Encouraging them to harm themselves.
It's not being a dick to speak the truth, no matter what these ironically myopic neopuritans think.