Rodgers and Hammerstein example occurs in Allegro, from 1947. The title song, reflecting on the hectic tenor of modern life: "Hysterically frantic, we're stubbornly romantic, and doggedly determined to be gay."
As late as 1961, there could be a Broadway musical titled The Gay Life without reference to homosexuality. A few later productions retitled it The High Life.
Cole Porter's 1934 musical, Gay Divorce, has nothing to do with gay couples divorcing. The film adaptation was titled The Gay Divorcee.
Who could forget the classic line from The Importance of Being Earnest: "The only way to behave to a woman is to make love to her, if she is pretty, and to someone else if she is plain"? (Being Oscar Wilde, it could have been dirtier than we think.)
Brigadoon has this dialogue after Tommy sings "The Heather of the Hill":
Fiona: Ye see. Ye can say nice things when ye want to. Tommy: Yes! It almost sounded like I was making love to you, didn't it? Fiona: Oh! There's a difference between makin' love an' jus' bein' sentimental because ye're tired.
Brigadoon also has a line in "My Mother's Wedding Day", describing just how blind drunk everyone was.
Everyone was blithe and gay at her mother's wedding day!
"Be firm, be firm, my pecker" in Trial By Jury. ("Pecker" means nose, as in the old saying "keep your pecker up," but modern audiences will assume something different.)
Patience has the title character, the only one of the maidens not to be swooning over Bunthorne, declare her ignorance of love: "For I am blithe and I am gay." Less reverential productions have the other maidens echo her line with a sneer: "For she is blithe and gay."
The Mikado also has "the nigger serenader and the others of his race" on Ko-Ko's list of people "who never would be missed". "Race", in the 1880s, meant something more like "type", "kind" or "sort" rather than "ethnicity", and "nigger serenader" refers to what we today would call "blackface minstrels" rather than actual dark-skinned performers.
Many an English class have found amusement in this play when Caesar's wife is interrupted by someone calling out (to the general crowd) "Peace, ho! Caesar Speaks!" Ho being a general "hey!" sort of word back in Elizabethan times...
Between Cassius and Brutus - "I have not from your eyes that gentleness/ And show of love as I was wont to have." and "Forgets the shows of Love to ther men." and "That I do fawn on men and hug them hard/ And after scandal them", "I cannot drink too much of Brutus' love."
Cassius: "..and Cassius must bend his body if Caesar carelessly but nod at him.", and (in reference to other Senators) "For who so firm cannot be seduced?", "Caesar doth bear me hard, but he loves Brutus."
Octavius: "Defiance, traitors, hurl we in your teeth". Ew.
Antony: "Tut, I am in their bosoms." (I can see into their hearts.)
Varrus: "So please you, we will stand and watch your pleasure." (We are content to watch.)
Brutus: "Lucius!/ My gown!", and "Get you hence, sirrah; saucy fellow, hence!".
Artemidorus, who writes Caesar a letter warning him of the conspirators, signs it, "Thy lover, Artemidorus."
Antony and Cleopatra must've been hilarious, given at one point Cleopatra faints and calls out "Help me hence, ho!" ... where she's saying so to her female servants.
In Baz Luhrmann's modernized version, William Shakespeare's Romeo + Juliet, the drag-queen Mercutio leeringly asks if Tybalt can spare him "a word and a blow." He also makes "the blind bow-boy's [i.e. Cupid's] butt-shaft [i.e. arrow]" sound as if it has something to do with bums.
In Henry V Act V Scene II, Princess Katharine explains in French to her fiancé that good girls don't kiss before marriage. At that point in time, the verb "baiser" meant "to kiss". It doesn't anymore. Considering that entire scene is mostly just for a Country Matters joke, Shakespeare probably would have approved of this.
The entire English Army is in a state of bedraggled gayness in Henry V. The Shakespeare quote is from King Henry V, Act 4, scene 3. In which King Henry is rejecting surrender terms offered by the French to his bedraggled tiny Army shortly before the Curb-Stomp Battle at Agincourt.
Let me speak proudly: tell the constable
We are but warriors for the working day;
Our gayness and our gilt are all besmirched
With rainy marching in the painful field.
Ellen Terry, an actress who played Beatrice (a character who is rather outspokenly against the idea of getting married for the first two acts) from Much Ado About Nothing had this to say about the difficulty in playing the part:
She must always be merry and by turns scornful, tormenting, vexed, self-communing, absent, melting, teasing, brilliant, indignant, sad-merry, thoughtful, withering, gentle, humorous, and gay, Gay, Gay!
In Shakespeare's A Midsummer Night's Dream, a character named Bottom has his head transformed into that of a hairy ass by a hobgoblin as part of an elaborate prank. Naturally, his fellow actors are horrified.
Given that "ass" was already being used as an alternative spelling/pronunciation of "arse", this was probably another deliberate one.
One Shakespeare's play has the line "Sir, give him head." It was an instruction to listen to what he had to say, not what you're thinking. It was probably a misspelling/variant spelling of "give him heed" or a reference to horseback riding, where "give him his head" still means "let him go ahead, let him do as he will."
The Tempest: Prospero, the rightful Duke of Milan, is usurped and put out of see on "a rotten carcass of a butt". 'Butt' here meaning boat.
As You Like It refers to the possibility of lovers being "incontinent" before marriage, by which the intended meaning is "cease from sexual abstinence", not "lose control of their bowels".
Happens over and over again in Hedda Gabler (translated into English in the 1950s). The word "gay" is used repeatedly as a euphemism between the characters for "overly hedonistic or sinful"... and the way they treat it as a euphemism just makes it more unintentionally funny.
In South Pacific, there are several uses of the word gay, as well as the lyric "High as a flag on the Fourth of July"
In The Diary of Anne Frank, Anne says that she made the shampoo she gave to Mrs. Van Daan for Hanukkah by mixing scraps of soap with toilet water. Here, "toilet water" refers to a type of perfume, but most modern audiences probably don't know this and find the line somewhat squick-y.
One of the several drinking songs in The Student Prince had a chorus starting, "Come, boys, let's all be gay, boys."
In Pal Joey, just before the Dream Ballet, Joey envisions himself becoming "the gay Joey." This has no connection to an earlier moment demonstrating that Joey likes to chase boys as well as skirts.
In the original Cyrano de Bergerac, after Cyrano has successfully negotiated Christian into Roxanne's room, the play notes "They begin to make love." Now, it is not clear what was meant by that in 1897, but in the movie, they are clearly making out like teenagers.
Eleanor Farjeon (best known as the writer of "Morning Has Broken") says she was five when she saw a play called "The Babes", a parody of Babes In The Wood. She remembers a group of soldiers singing "We are Gay Volunteers! How we splash! How we dash!", apparently in reference to their fancy uniforms and not to the fact that Bertie, the heroic Captain of the Volunteers, was played by Miss Grace Huntley.
The one-act play "Yesterday" contains an example of "coming out" referring to young debutantes entering the social circle. The main female character, Lady Ann Trevers, is an elderly lady at a Great Gatsby-style party; she complains to a man of similar age: "These coming out parties are not what they used to be."
A Polish translation of Molière's The Miser has a police officer saying "Leave everything to me" in the way that nowadays means "Cum on me", and middle-schoolers love it.
"Life is earnest, art is gay." from German playwright Friedrich Schiller's Wallenstein's Camp (not that Camp).
The song "Blow Gabriel Blow" from the 1930's musical Anything Goes. It's in the style of a gospel song, but the chorus and some of the lyrics sound filthy to modern ears. Since the musical is a comedy it fits right in.
Thomas Middleton's The Witch has two examples—one a word and one a phrase—in the following line about a charm for a potion:
Hecate: ...Titty, Tiffin, keep it stiff in.
The Biograph Girl has the song "I Just Wanted to Make Him Laugh," sung by the character of Mack Sennett. Sennett, a comedy actor, laments the fact that the humorless director he was auditioning for had no appreciation for his comedy skills with the line, "Perhaps given another time and place / he'd have taken me on his staff." And this is after he sings the somewhat more innocuous line, "I'm not a straight dramatic actor." Suffice it to say that he succeeded in making us laugh. What makes it even better is the fact that Sennett was played onstage in London by Guy Siner, known for portraying the delectably gay Lieutenant Gruber.(I'd take him on my staff... anytime!)
In Hamlet, Polonius advises his son Laertes "This above all: to thine own self be true, And it must follow, as the night the day, Thou canst not then be false to any man." Where modern theater goers might interpret this as advice to Be Yourself, Polonius was actually warning Laertes to look out for number one, asserting that if Laertes is in good standing he can then afford to spread the love to his fellow man.
Earlier in the play, in "The King Of Old Broadway", Bialystock lampshades this trope when he sings, "There was a time when I was young and gay—(beat)—but straight." Who was originally played by Nathan Lane, who's very much out, adding another layer.
From Angels In America (itself subtitled: A Gay Fantasia on National Themes, this itself not an example):
Prior: "I'm gay."
Prior's ancestor: "Well, be gay, dance in your altogether for all I care, what's that to do with not having children?"
Prior: "Gay homosexual, not bonny, blithe and... never mind."
There's a scene in a play I saw a long time ago that I think was called "Young Rube" which was about the life of Rube Goldberg, and his imaginary friend Boob Mc Nutt. Rube at one point tells of Boob's antics to a friend, who goes out and tells... a bar full of sailors, "Everyone should listen to their Boob!" As expected, everyone suddenly tries to lean their head down to their chest.
The Fantasticks has (or had) an intentional example, where El Gallo, hired by the heroine's parents to kidnap her (It Makes Sense in Context), refers to the kidnapping as "rape", at which the parents are kind of freaked out until he explains that he doesn't mean it in that sense. A number called the "Rape Ballet" ensued. Later productions, however, have changed this to the "Abduction Ballet".
There was (and still is) a classic toy kit full of miniature girders, bolts, and platforms, primarily marketed to young boys; an "Erector Set". The Firesign Theatre caught that over 30 years ago: