Are you an only son or only child? I only ask because as a mom of sons in the states we are socialized to believe whether it's entirely true or some broad generalization that sons grow up to leave their mom. So basically whoa is me for not having daughters?? It could just be wives tales who knows? I'll personally cross that bridge when I get there.
Maybe subconsciously the other sees threats? Threats = fear. I'm not sure why it's necessary to completely wash hands of your own family in your gf's eyes? I personally wouldn't dream of asking a man to not have anything more to do with his family. And I could personally learn to tolerate occassional etiquette as a compromise, hence not understanding fully nor comprehending such willfullness as your gf sounds like she is displaying.
Unless, I'm not understanding? You'd live seperately from either family, would you not?