Not always a great rule of thumb, considering vaginal size differs from person to person as does the size of the penis.
I'm sure a few not-so-well-endowed males have followed such a standard and come to the conclusion that they live in a world of sluts. Of course, some might reach that conclusion regardless (*cough* OV *cough*).
Most girls will have more than one sexual partner in their lifetime. If, for some reason, you really want to stick with this girl, it's still probably good for you both to get a checkup before continuing to have sex.
~Sphinx