I believe I would have to act if my wife got randomly groped by some drunken weirdo. I think I would respond verbally at first, try to make the guy take the first swing, and then do my best to defend my wife’s honor. I couldn’t live with myself otherwise. Just standing there like an idiot why some jack-off does possibly the worst thing he can do to my wife? Yeah, no. Whether or not I get my ass handed to me is irrelevant. Mr. Gropey is going to get everything I got.
If the police happen to be right there, I would certainly seek their assistance instead. But that would be unlikely.
Honking on a boob that doesn’t “belong to you” is a cardinal offense, IMO.