Just gonna throw this out there but i reckon theres mistrust out there already.
Just because someone lives with the opposite sex means things will happen.
In my second year we lived in a mixed house and noone slept with each other, many had other partners and everyone lived together as mates... i know many people that have lived with people from the opposite sex and noone has had an issue, so i dont see why this should be?
Plus everyone says theyll live with so and so in the first few weeks of uni as they still live in a bubble, this quickly changes over time.
I think you need to work on your trust (or possibly lack of) before you worry about who your girlfriend lives with as you dont really get a say lets be honest. She can live with who she pleases.