I think one thing that gets parents antsy is not their adult kid having someone sleep over it's the someone then hanging around all day, eating their food, being one more person in the bathroom, basically just treating it like their house. If you have a small house especially this can be quite an invasion. They might like your gf/bf fine but that doesn't mean they want to be talking to them in the kitchen at 7am or waiting for them to get out of the bathroom.
So if you keep in mind the possible friction of invading what is your parents space with another person they might not object to her staying over.
And I second the floor, if the bed makes noise. Or put the bed mattress on the floor.