Conclusion
Joan's is one of the most complex fantasies I have come across in years. Yet there does seem to be an answer for almost every part of it.
The long hall that she walks down at the beginning is reminiscent of the long walk she took through the deserted house, just to find the room where she would lose her virginity. Her exhibitionist trait has probably always been inside of her, but it was brought out quite strongly when she saw the mice witnessing her first experience.
After that, she always wanted to be seen, preferably by human beings.
All exhibitionist get their thrills when they know that they are arousing the voyeur who is watching them. Joan is no different. The thing that turns her on the most is knowing that the men who watch her are hard.
Finally, there is the most important aspect, which is the force that is used to make her perform. Whether she is tied or just being made to go down, there is still force involved, and it's easy to tell that the force is necessary so the girl doesn't feel guilty about what she's doing. She talks of guilt a great deal, especially after talks with her mother. A girl can always justify a forced entry (and feel no guilt) by telling herself that she didn't want it but can't do anything about it.
That seems to sum Joan up quite well.