I wouldn't have expected you to be.
This is the kind of dumb@$$ thing that gives educators a bad name and would make me not want to admit I was one (Just like the *meteorologists* who claim global warming is a fact. I can't help what others think or say. *sigh*)
I took him out in the hall and told him I knew what that meant and made him wash his hands off with soap and water.
I told the AP in case he did it again and called his mother that evening using the TTY service. She was flustered and I told her he wasn't in trouble this time and we ended up having a good laugh over it.
He came in the next day and told me His mother was making him apologize to me and the class.
No fuss no muss and never another problem.