It's often repeated in this community that mate guarding in public is a sign of weakness. This is certainly true in most cases, but I wonder if there are also circumstances where it would be a sign of weakness to not react.

For instance, if you're in a club and a dude walks up to your girl, slaps her ass, and puts his arm around her, what should you do? Acting indifferent in a situation like that seems counterproductive. What is the consensus for stuff like this?