In another question, the whimsically yclept Yoichi Oishi asks
What does "Without padding one’s end zone" mean?
and Robusto replies,
'End zone' is a pun, because the object of American football is to have one of your team carry the ball across the opponent's goal line into their end zone.
I'm thinking, no, it's not a pun. Calling a football fan's buttocks his end zone is just an ad-hoc slang phrase, since padding the end zone is not, I believe, something done in football.
By comparison, imagine you were at a football game and another fan said in reference to a comely cheerleader, her back-field is in motion. Again we have a metaphorical reference to someone's bottom (in football, the back-field is all the offensive players who do not start at the line of scrimmage) but there is a pun, since back-field in motion is a real football term (meaning, I believe, one of the back-field players is already moving when the ball is snapped).
So, is my understanding of football too weak or my definition of pun is too narrow?