An interesting little history by Wayne R. Dynes, which briefly goes over several writers throughout history promoting this misconception. Sample:
The scholastic theologian Albertus Magnus (d. 1280) held that the vice of sodomy was “more common in persons of high station than in humble persons.” This impression reflects in part the greater visibility of the doings of the privileged, and also the fact that, through their status or influence, the nobility could frequently escape with a reprimand for the commission of crimes which were subject to capital punishment when committed by commoners. This aspect of class justice has fueled social envy, leading to the demand on the part of the straitlaced middle class that the aristocracy be disciplined and required, for its part, to adhere to the narrow canons of petty bourgeois morality.
Yes, it's a medieval facet of celebrity culture, just as today celebrity coming out is a big media story and if a janitor do so, nobody cares except his family and friends (and priests of course which was much bigger problem back then obviously when they held power over people lives). Also "middle-class" being bigots is a story as old as humanity.