A gay person, who may or may not be out, and actively works against the interests of the LBGT community. Like an Uncle Tom for the gays, he will go along with the conservative right wing rather than growing a pair and standing up for himself and others like him. Exhibiting high self deception, he's acting overly anti-gay to avert suspicion that he is gay. Often applied to a politician.
By voting against every gay friendly piece of legislation he could, Larry Craig outed himself as Uncle Mary.
The California State Senator caught driving while intoxicated when leaving a gay bar with a male passenger, Roy Ashburn, is a sad Uncle Mary.