The word gay in the sense of homosexual seems to have come into use somewhere in the 1930s or 1940s for reasons that are obscure. It was an improvement over the various terms that preceded it, but the choice was not a happy one.
In the first place gay in the original sense of lighthearted and debonair seems no more applicable to homosexuals than to anybody else, and in the second place people rarely use it in that sense anymore for fear of being misunderstood or snickered at.
The result is that we have virtually replaced a lovely old adjective with a peculiarly misleading one and incidentally ruined some of the best lines W. B. Yeats ever wrote, in which he said of two old Chinamen that "Their eyes mid many wrinkles, their eyes, / Their ancient, glittering eyes, are gay."
It is nice to have more or less gotten rid of the likes of faggot and queer, but one can only hope that eventually, along with everybody else, homosexuals will be referred to simply as human beings.
~originally published in Beyond Words