One Thing Led to Another

""Oh, and stay away from your friend the Captain. You save his life, he saves yours, one thing leads to another... [meaningful pause] I was young once too.""

- Dr. Sid, Final Fantasy the Spirits Within

The pattern is always the same: Two minor, innocuous incidents, and then the whammy: "...and the next thing I knew, we were having sex."

The most popular way to brag, to confess to someone you've cheated on, or to answer an angry parent who has asked, "How did this happen?", as a way to avoid going into all the details no one really wants to know. If this is how someone (optimistically) plans to get someone into bed, it's Step Three: Profit - Step One, Boy Meets Girl; Step Two: ???; Step Three: Coitus Ensues!

Don't Try This At Home.

Anime and Manga

 * In Dragon Ball Future Trunks sums up Bulma and Vegeta's relationship as "...and then she saw my father sitting there so proud and lonely and then one thing just led to another."
 * Fate/kaleid liner Prisma Illya 2wei chapter 16. Judging by Luvia's Imagine Spot, she hopes to eventually have a child and marry Shirou using this method.

Literature

 * In A Song of Ice and Fire, Robb ends up breaking a marriage treaty because of this, which has very unpleasant consequences later (just like everything else in the series).
 * This is how Rand Al'Thor falls for two of his three love interests.
 * Decorated with numerous lampshades in William Goldman's early novel Boys and Girls Together, where Jenny actually says at one point, "In a book it'd say 'their lips met' and then there'd be a double paragraph break and a line of stars," whereupon-- their lips meet, and there's a double paragraph break and a line of stars to imply that the characters had sex.
 * A non-humorous version in Harry Turtledove's second Worldwar novel Tilting the Balance, as 's explanation to of how  and  first ended up sleeping together in the first novel. The phrase is used verbatim.

Live Action TV
"Meredith Rice: He was crying. I followed him to make sure he was okay. One thing led to another... Next thing I know, we're having sex. Detective Stabler: And that seems like a perfectly natural segue to you?"
 * Frequently heard on Law and Order Special Victims Unit. A pregnant teenaged girl pleads, "We never meant for it to happen, it just did." An ex-boyfriend recalls, "We shared a glass of wine, one thing led to another, next thing I knew, we were in bed." A teacher who raped a 12-year-old boy defends herself:

"Pete: I met this girl and, and one thing led to another... Bronson: What he's trying to say is, he got her pregnant! Pete: No! She got me pregnant!"
 * Used by the model six Cylon in Battlestar Galactica: Razor in describing how she got involved with.
 * Round the Twist, when Pete tries to explain to his father the strange supernatural twist of the week.

"George Costanza: "So Listen to this. Marcy comes up and she tells me her ex-boyfriend was over late last night, and "yada yada yada, I'm really tired today." You don't think she yada yada'd sex." Elaine: (Raising hand) "I've yada yada'd sex." George: "Really?" Elaine: "Yeah. I met this lawyer, we went out to dinner, I had the lobster bisque, we went back to my place, Yada Yada Yada, I never heard from him again." Jerry: "But you yada yada'd over the best part!" Elaine: "No, I mentioned the bisque.""
 * Hunter. An ex-girlfriend describes to Dee Dee McCall how she once got involved with Hunter by saying, "This lead to this, that lead to that..."
 * In an episode of Friends, it is revealed that when Ross was in high school, he had sex with the school librarian. When asked how it happened, he replies: "I was working late in the library one afternoon. It was just the two of us. She needed some help with her word jumble. And one thing led to another..."
 * In the Seinfeld episode "The Contest", George uses the phrase to describe how he ended up masturbating in his parents' house: "I stopped by the house to drop the car off, and I went inside for a few minutes.. Nobody was there - they're supposed to be working. My mother had a Glamour magazine, I started leafing through it... So, one thing lead to another..."
 * Seinfeld also uses a similar phrase, "Yada Yada Yada," to describe a mysterious lead-up to strange or sexy events.

"Will: And then she slipped into something more comfortable, and she's showing me pictures of San-- San something, and then, you know, before I knew it... you know... Ta-dow.
 * In The Fresh Prince of Bel-Air, Will gets seduced by his girlfriend's mother, while she was trying to start a fling with Uncle Phil. To keep the same from happening to Phil (caring, not cock blocking), Will takes Phil aside to warn him:

Phil: Tadow?

Will: You know, "taaaa-DOW".

Phil: ''"taaaaaaaaaaaaa..."-oh!-OH! YOU SLEPT WITH JANICE?!"

"Leonard: Okay, let me try it this way, uh, I was at a coffee klatch with a couple of friends and one thing led to another and it turns out you’re off the team."
 * The Big Bang Theory

Music

 * Insane Clown Posse, "Please Don't Hate Me": "One thing just led to another/Next thing I knew, I'm butt-fucking your mother!"

Stand-Up Comedy
"I hate when you're trying to read something and you come across the expression "One thing led to another". What in the hell kind of lazy writing is that? Isn't that your job as the writer to tell me how this led to that? You can just throw that in there? "Adolf Hitler was rejected as a young man on his application to art school. One thing led to another...and the United States dropped two atomic bombs on the sovereign nation of Japan." "This is some pamphlet!""
 * Brian Regan:

Theater

 * "...And one thing led to another, and dot dot dot !!"

Video Games

 * Rose says this about her relationship with  in Metal Gear Solid 4.

Web Animation

 * Parodied in the Homestar Runner toon "Pumpkin Carve-Nival", where the King of Town tries to explain how his Jack-O-Lantern entry turned into a partially-eaten pumpkin pie.

Western Animation
"Amy: Well, the car broke down on Mercury, and one thing led to another...
 * In an episode of Futurama:

Fry: ... and then it led there again when we got back home"