No, you aren't stupid, but what that man did is inexcusable and lewd. Sorry you had to go through that. Having been in my fair share of mosh pits and groped by my fair share of strange hands as a result, I can sympathize.
Unfortunately, there is a risk that germs could have been passed to your hand from the mess. That risk isn't really high, but it's not negligible either. What you can do at this point is tell your doctor what happened and if he/she deems it appropriate, you can get a blood test to see if you might have picked anything up.
And in case you haven't laundered your jacket just yet, ask your local police dept. if there's anything you can do as far as criminal charges go (they might be able to extract DNA for identification purposes).
Universal Law #4721: Any joke can be made funnier with the word "goatf**ker." Corollary, any story can be made more entertaining by adding a few ninjas.