How did "gay" come to mean "homosexual"?
Where, when, how, etc., did the good-natured word "gay" pass into the vernacular as a designation for all things homosexual? Can one be homosexual without being gay, and vice versa?
Hate to tell you this, Tom, but the "good-natured word 'gay'" has been leading a double life. Although many people believe "gay" simply meant lighthearted or cheerful until it was shanghaied by the preverts, the truth is the word has long had a secondary connotation of sexual licentiousness. As early as 1637 the Oxford English Dictionary gives one meaning as "addicted to social pleasures and dissipations. Often euphemistically: Of loose and immoral life" — whence, presumably, the term "gay blade." In the 1800s the term was used to refer to female prostitutes; to "gay it" meant "to copulate."
By 1935 the word "geycat," meaning a homosexual boy, had found its way into print, giving a clue as to the direction things were starting to go. Sure enough, by 1955 "gay" had acquired its present meaning, as P. Wildeblood notes in Against Law: "Most of the officers had been "gay' … an American euphemism for homosexual." Actually, gays had probably been using the term among themselves long before.
Ghettoization of the term began to occur in the 60s so that today "gay" in the sense of "homosexual" has chased out all other uses of the word. This is more the result of the squeamish attitude of the straight world than any organized campaign on the part of gays, and in any case it's no big deal; there are plenty of other words that cover the same territory that the non-sexual meanings of "gay" did.
At one time "gay" referred strictly to male homosexuals; female homosexuals were called lesbians. (This distinction may no longer hold true — today one hears lesbians being referred to as gay.) Whether all male homosexuals would consent to be called gay — whether, for the matter, all gays would consent to be called homosexual — is a question I will not presume to answer. I am quite certain, however, that most gays would reject the implication that "gay" necessarily implies promiscuity.