It is rude to doubt the ability of others.
Mark is the head of the swim team, and a black man wants to join.
Mark wants to say something to the man.
Mark asks the man what swim stroke he is best at.
Mark ask the man if he is sure he wants to join the team since he can't swim.
The man feels respected and is happy that Mark doesn't judge him.
The man is insulted and is no longer interested in joining the team.