Lynn, I think that in the big picture you handled this in the best and most mature way you could. I can't say that I know exactly why the parents reacted as they did. i can guess, and the possibilities range from AAAA*S to the worst case scenario. I think that right now it is very hard to tell exactly what is going on.

I know that I had to talk to a woman I know about her son touching iappropratiely one time at a party at my house. We were able to handle it well, but i wondered where he had gotten it from. It turned out later that he had an undiagnosed ADHD, and one of the things we now look for with both ADHD and bipolar kids is so-called Precocious sexual behavior. Even if things are going OK at home or school, these kids pick societal and media signals and act on the impulses they catch from those because they can't screen them out. This can be very bad, because the first suspicion is always going to be turned on the family (naturally and usually correctly) or an external abuser (again naturally and usually correctly). You can imagine the affect this has on the family, even if the parents are innocent. The defensiveness must get absolutely pathological in its own right.
Personally, they were out of line on quite a few points, but let the school deal with it, stay distant and if youhave to deal with them, civil. I think you are doing great. Huggs!