Even though I'm not gay it really bothers me when people use the word gay as a negative term.
When I was little it didn't bother me. When I began to have strong politically correct attitudes toward race/sex/orientation/etc. in my early teens I was bothered by it. When I came to realize that the word had existed for about 90% its history not meaning "homosexual" but rather "happy" I became annoyed that anyone would take issue with it. My reason for saying this is pretty simple.
Imagine if the "preferred" term for the mentally challenged became "goofy" and then after a few decades of "goofy" meaning both its traditional definition and the new context for referring to mentally challenged folks there suddenly was some crusade to solely use "goofy" for ironically the context its been used for a shorter period of time than any other meaning.
I don't think it'd be cool calling them "goofy" but I figured it was similar to the absurdity of calling someone a synonym for "happy" regardless of their temperament as is the case with gay.
Also, if the meaning of "gay" has suddenly been reserved for formally referring to a homosexual then it seems hypocritical for folks to just expect the word to "evolve" into its current usage but have a problem with it "evolving" into use in another context especially when its use in the homosexual context only accounts for less than 10% of the time it's been established as a word.
https://en.wikipedia.org/wiki/Gay#Overview
The word "gay" arrived in English during the 12th century from Old French gai, most likely deriving ultimately from a Germanic source.[1] For most of its life in English, the word's primary meaning was "joyful", "carefree", "bright and showy", and the word was very commonly used with this meaning in speech and literature. For example, the optimistic 1890s are still often referred to as the Gay Nineties. The title of the 1938 French ballet Gaîté Parisienne ("Parisian Gaiety"), which became the 1941 Warner Brothers movie, The Gay Parisian,[6] also illustrates this connotation. It was apparently not until the 20th century that the word began to be used to mean specifically "homosexual", although it had earlier acquired sexual connotations.