It's not expected that you put ketchup on ham.
Dennis is on a date at a fancy, upscale restaurant.
Dennis wants to enjoy the ham he ordered.
Dennis takes a bite of his ham and enjoys the way the chef cooked it.
Dennis asks the waiter to bring some ketchup and pours some on his ham.
Dennis has a nice evening with his date.
The chef gets offended and Dennis' date thinks he's weird & low class.