It really depends on the size of the plug, if you take it as punishment. Most wouldn't be, as they can be very sensual and can arouse easily, but not give much pain. Now, if you had one that was much larger than you've ever taken before (Say, you now take 3 inches wide instead of one inch) then it would probably be considered a form of punishment if left in for a long period of time.
As I just said, the easiest way to make it into a punishment is to take the plug, make it larger than your sub, or you has taken before, and be forced to leave it in there for a certain amount of time to make it unbearable.