Probably in the late 60's

I wonder that too, maybe Cary Grant knows.

Because if someone were to say, I'm gay. Also, you're gay, another individual would assume different, just because it could mean something else. So after that, I guess most people take it that kind of way, or else they would just say happy instead of gay.

gay means happy they chose the word themselves'

gay 1178, "full of joy or mirth," from O.Fr. gai "gay, merry," perhaps from Frank. *gahi (cf. O.H.G. wahi "pretty"). Meaning "brilliant, showy" is from c.1300.OED gives 1951 as earliest date for slang meaning "homosexual" (adj.), but this is certainly too late; gey cat "homosexual boy" is attested in N. Erskine's 1933 dictionary of "Underworld & Prison Slang;" http://www.etymonline.com/index.php?search=gay&searchmode=none

Sometime during The Flintstone's original run on television.

This adjective, meaning joyful or light-hearted, is of uncertain origin. The English word comes from the French gai, but where this French word comes from is uncertain. There are cognates in other Romance languages, notably Provencal, Old Spanish, Portugeuse, and Italian, but no likely Latin candidate for a root exists. The word may ultimately be Germanic in origin, with the Old High German wâhi, meaning pretty, and gâhi, swift, being suggested as possible progenitors, but the transition from the medial h in those roots to the Romantic forms is problematic. The word is first recorded in English sometime before 1310 in a poem found in T. Wright’s Specimens of Lyric Poetry: Heo is...Graciouse, stout, ant Gay, Gentil, jolyf so the jay. (He is…Gracious, stout, and Gay, Gentle, jolly as the jay.) Another early use is in Chaucer’s The Miller’s Tale, c.1386: This Absolon, that iolif was and gay, Gooth with a sencer on the haliday. (This Absolon, that jolly was and gay, Went with a censer on the holiday.) In recent years, however, the traditional sense of gay has been driven out of the language by the newer sense meaning homosexual. Many believe this new sense of gay to be quite recent, when in fact it dates at least to the 1920s and perhaps even earlier. This early existence is as a slang and self-identifying code word among homosexuals, only entering the mainstream of English in the late 1960s. So how did this word meaning joyful come to refer to homosexuality? There are two, not necessarily mutually exclusive, commonly proffered explanations that are plausible. Perhaps the most commonly touted one is that the modern use of gay comes from a clipping of gaycat, a slang term among hobos and itinerants meaning a boy or young man who accompanies an older, more experienced tramp, with the implication of sexual favors being exchanged for protection and instruction. The term was often used disparagingly and dates to at least 1893, when it appears in the November issue of Century magazine: The gay-cats are men who will work for “very good money,” and are usually in the West in the autumn to take advantage of the high wages offered to laborers during the harvest season. The disparaging sense can be seen in this citation from the 10 August 1895 issue of Harper’s Weekly: The hobo is an exceedingly proud fellow, and if you want to offend him, call him a “gay cat” or a “poke-outer.” And from Jack London’s The Road, published in 1907, but this passage is a reference to 1892: In a more familiar parlance, gay-cats are short-horns, chechaquos, new chums, or tenderfeet. A gay-cat is a newcomer on The Road who is man-grown, or, at least, youth-grown. The second possible explanation is that the homosexual sense is an outgrowth of an earlier sense of gay meaning addicted to pleasure, self-indulgent, or immoral. This sense dates to at least 1637, when it appears in James Shirley’s play The Lady of Pleasure: Lord. You’le not be angry, Madam. Cel. Nor rude, though gay men have a priviledge [sic]. By the early 19th century, this sense had developed into a euphemism for prostitution. From John Davis’s 1805 The Post-Captain: As our heroes passed along the Strand, they were accosted by a hundred gay ladies, who asked them if they were good-natured. “Devil take me!...there is not a girl in the Strand that I would touch with my gloves on.” This could easily have transferred to male prostitutes and then generalized to mean homosexual writ large. One potential early usage of the modern sense of gay, meaning homosexual, is from an 1868 song by female impersonator Will S. Hays titled, Gay Young Clerk in the Dry Goods Store. The lyrics do not explicitly link the word with homosexuality, but they can be interpreted that way, especially if sung by a man in drag. Then again, that may be reading too much into the lyrics. You be the judge: It’s about a chap, perhaps you know, I’m told he is ‘Nobody’s beau,’ But maybe you all knew that before, He’s a lively clerk in a Dry-Goods Store. O! Augustus Dolphus is his name, From Skiddy-ma-dink they say he came, He’s a handsome man and he’s proud and poor, This gay young clerk in the Dry-Goods Store. Another early appearance that is of questionable meaning is from Gertrude Stein’s 1922 Miss Furr & Mrs. Skeene which appeared in Vanity Fair. It is uncertain, however, if Stein’s use of gay in this case is a reference to lesbianism or to the conventional sense of gay meaning happy: They were...gay, they learned little things that are things in being gay,...they were quite regularly gay. The first unequivocal written use of gay to mean homosexual is in 1929, in Noel Coward’s musical Bittersweet. In the song Green Carnation, four overdressed, dandies sing: Pretty boys, witty boys, You may sneer At our disintegration. Haughty boys, naughty boys, Dear, dear, dear! Swooning with affectation... And as we are the reason For the “Nineties” being gay, We all wear a green carnation. The penultimate line refers to the 1890s, which were commonly called the gay nineties. In general usage, this appellation had nothing to do with homosexuality, but in this context, Coward clearly uses it as a double entendre. It appears again in Charles Ford and Parker Tyler’s 1933 The Young and Evil: Gayest thing on two feet. An in the 1938 movie Bringing Up Baby, the character played by Cary Grant, when asked why he is wearing women’s clothing replies: Because I just went gay all of a sudden. This is obviously a joke intended to slip past the censors. The term remained slang within the homosexual community until the late 1960s, when the Stonewall riots and the rise of homosexual rights activism brought this sense of gay to wider society. It’s probably worth mentioning that there is a false acronymic origin for gay floating about, that it stands for Good As You. Like most acronymic origins for words, this is just incorrect. http://www.wordorigins.org/index.php/site/comments/gay/

Probably for the same reason that "Wicked" "Sick" and "Stupid" also means "good" now days.

because thats the way fucked up people are you fucking cock sucker

It was a slow process that pretty much was over by the 1960s