oh i don't know, being called a slut by a stranger i'd just ignore. With friends [because they did call me that often, i must say] i'd just continue the game, probably with a light growl--all fun and games of course, nothing sexual. So, either from a stranger or from a friend being called anything doesn't affect me at all--unless it's things i as a person find hurtful, i.e. i dunno, being called a cow.
Now, coming from a sexual point of view, it plain and simple turns me on. Even in a D/s relationship where it's not only sexual, it will still bring out my more submissive side.
But what a slut is? I honestly don't know, i mean my grandmother would probably give you a completely different definition that my own. :s