If you honestly think it's appropriate to be violent because a man looks at your junk then you probably need to chill out. It's gross and rude but so is staring at a woman's cleavage or at her breasts or camel toe at the beach. Neither situation warrants violence.
Violence is not appropriate, neither is it for a guy to check out a strangers junk.
We can't control how others act, and if they do become violent, realize the small part you played by looking at his privates and violating his right to privacy in a bathroom.