Why can't she behave like a normal girl?

Dear Diana,
My girlfriend is a very badly mannered girl. She screams at her parents, she burps while she eats (and she CAN eat!), even farts in public and in general, behaves like a tomboy. I've tried to drill some sense of what accepted feminine behaviour is supposed to be, sound or look like, but she just won't take a hint. In fact, she even called me sexist. God knows I'm not, it's just that this kind of behaviour is very off-putting. Help!

Yatin