She can't stay if they aren't home.?

So I'm almost 21 and my lovely girlfriend is turning 20 here pretty soon. My issue is that my parents insist that we can't be home alone together if one of them is not also home. They say they don't want us to have sex, but they know full well that if we want to we could just go outside and do it or what-have-you. Anyway, we are pretty straight laced, never any drugs or alcohol, we make straight As in college. How do I get my parent to chill out? They constantly tell her that she is welcome over any time and then she will get there and they will text me saying that IIIIII need to tell her to leave. Help!