Illegitimate enjoyment. Saying that a game is "bad wrong fun" indicates that the game is somehow broken, unplayable, poor quality or weak - in ignorance of the fact that many groups are playing and enjoying it regularly. As such, this is not normally something that a person will say themselves, but something they will accuse other people of having said. For example, "John said that d20 is bad wrong fun" indicates that the speaker believes John's criticism of d20 to be foolish given the large number of groups playing and enjoying it. Rifts is often cited as a "bad wrong fun" game. Often written as a single word, badwrongfun.
"Bad wrong fun" is also often used in a more light-hearted sense to indicate the gamer equivalent of a guilty pleasure. "I know it's bad wrong fun, but I loved every minute of it!"
The earliest reference I can find to its usage is from RPG.net in 2004.
It is clearly a concept that is not peculiar to role playing games. For example, football matches could be badwrongfun for people who prefer opera (or, of course, vice-versa). In a more general sense, it encompasses a concept where the enjoyment of one social group is derided as not "proper" by another social group.
This is not a "guilty pleasure" because that implies that the participant themselves considers that there is some sort of illicitness in the pleasure. To the participant this is a perfectly acceptable way of having fun, it is only the observer who finds it offensive.
Is there a general term or phrase for this phenomena? Alternatively, are there other phrases that encompass the concept in other social settings?