There could be other reasons. Vaginal dryness (not a problem I have, but it does occur), fear of pregnancy, maybe even an abnormal Pap test. Or maybe they don;t want to tell you they have genital herpes.
Those are valid points you make- I do know she would kind of resist, when I would try to spread her legs with my hand but it wasn't a problem when I penetrated her with my dick! Tried to find out what was up but never really got a straight answer! So I didn't push it. Maybe she had a real bad experience of some kind too!