I wanted to chime in on this one. Just because the OP thought the BIL was taking only the licorice, doesn't mean he didn't possibly touch other parts of the cupcake(s). I don't get too icked out by germs (in my own house with my own immediate family), but if I had been present at that gathering, I would have made a point of backing the OP up by excusing myself and my kids from having the dessert and, hopefully, would have been able to explain it her afterwards. Sometimes the point sinks in better when there is a 'crowd' making it.
As for the DH, maybe he would understand how the OP feels if she were to (hypothetically) pick the onions and mushrooms off his steak or the yummy, crunchy crust off his baked chicken. This behavior could be used on any number of foods, not just desserts. Maybe using wetter foods as an example would drive the point home to the DH about how rude and unsanitary and just plain yucky it is to do such a thing.