Stormbenders uses the term "pearl diving". When a man asks a woman if she would like to participate, it has nothing to do with oysters. Incidentally, the town that the main characters reside in is called "Oyster District"; this term gives the town name a whole new meaning. "Pearl diving" was also used as a euphemism for underwater masturbation in the book Haunted 2005.
Some of the joy of reading Fallout: Equestria comes from Littlepip's creative swearing, with such gems as "Luna shitting moon rocks!" Hell, Celestia knows her as "she of the colorful vulgarities," to Littlepip's profound embarrassment.
Narration: Draco and Ebony removed skin from an onion. After that, they put Draco`s noodles into Ebony`s bowl, and Draco then planted some seeds into Ebony`s pot, and then they laid on their bed for the rest the night. They were very tired from all that hard work.
Wings To Fly has a few odd euphemisms from the pilots involved. A "Pilot's Kiss" has absolutely nothing to do with kissing; it's a reference to an x-shaped chest bruise from the four-point harness standard to most mobile suits. Similarly, when someone tells you "foxtrot tango" what they actually mean is "I can not or will not obey your order", though it would be most literally translated as "fuck that".
While Bella was the most affected, the brothers always to advantage and made lingering touches, and innuendoes such, "my meatballs are bigger then this and tastier" courtesy of Rabastan and "My breadstick is longer and thicker" courtesy of Rodolphus.
What sent Bella over this time was when her lovers dipped both of their breadsticks in her pasta, saying "We wanted to taste your sauce."
Snape: Not too long after we graduated from Hogwarts, I realized what a fool I'd been in letting Lily leave me on such horrible terms. I managed to seek her out before she and James went into hiding, and I convinced her to forgive me.
Snape: Then she forgave me again the next morning.
In A Necessary Gift Sirius comments that he hopes to get lucky at a Beltane ceremony. Remus objects to this remark because a ten-year-old alternate-universe Harry (who's pretending to be Sirius' nonexistent son Orion) is standing right there.
Sirius: Orion's old enough to know about the quills and the inkpots, Remus.
In At The End of All Things Ginny sees Hermione and Luna kissing and briefly has a negative reaction due to her mother's derogatory comments about "cauldron-licking girls."
In Dearest Hermione Sirius tries to give Harry The Talk, due to his doubts that the Dursleys would ever have discussed "the flowers and the fairies."
Harry: Something smells really strange and I can't figure out what it is. It's not a bad smell, just a really strange one. Fred Weasley: I don't smell anything strange, but I just had a sudden and unexplained goblin uprising in my trousers.
A particular (pre-clonebomb) fanfic somewhere in the Homestuck fandom has Jade decide that "tangle buddies" is an appropriate name for nervous interlacing of fingers, which then gets seized on for some reason as a euphemism.
"Oh," Jade interrupted, "I already knew that." [...] "But now you're tangle buddies, which is even better!"
Hivefled takes advantage of Bizarre Alien Biology to come up with these; "bucketdrinker" is a troll equivalent of "slut", and "gutbucket" is their answer to "cunt" (more specifically, it refers to the pouch in which genetic material was carried before the invention of buckets). "Pailing" as a term for reproductive sex is also used, as is common in the Homestuck fandom. Not that they don't liberally use the human words as well. After Dave sees troll porn, tentabulges become "dayglo banana slugs".
One of the villains in The Private Diary of Elizabeth Quatermain speaks of his upcoming marriage as being the means to an end, saying that "I just have to take the blush off your English rose" in order to achieve it. As he's talking to the man who is genuinely in love with the bride, this is quite possibly the worst pressing of a Berserk Button in the entire series.
The Ballad of Twilight Sparkle gave us the somewhat memorable statement that using magic with an antimagic collar apparently "hurts like an applebucker". Admittedly, given that "apple-bucking" in My Little Pony: Friendship Is Magic consists of kicking a tree hard enough to rattle all the apples out of it in one blow, it seems reasonable that being bucked would indeed hurt a lot.
This is the Life: A Tale of a Human in Equestria has the main character (a human) use terms like "pony women" instead of "mares" and of course "everyone" instead of "everypony" while the ponies use the correct (At least for Equestria) terms. Word of God claims it's to cement the fact that the titular character knew absolutely nothing about ponies or their way of life before moving there. The ponies also sometimes get messed up when referring to the main character:
Big Macintosh: Ah knew you of all pon-, uh, you’d put it to good use.
Dash Academy mentions the "wingboner" (when an excited pegasus has their wings spread open involuntarily), which is a common fandom term and not really a euphemism at all (indeed, it is really a dysphemism, as pegasi spread their wings for a lot of reasons). However, later on when Rainbow Dash falls asleep in the theater and has a dream about kissing Firefly, her spread wings make Firefly refer to Rainbow Dash as having had a "Night Flight", an obvious euphemism for wet dream.
In the My Little Pony branch of The Infinite Loops, they swear to trees, instead of the usual swearing to Celestia in My Little Pony, which is itself an example of Unusual Euphemism
Celestia: I want you to stop swearing by me. ... It really is nothing personal, but when you can't tell if 'dear sweet Celestia' is an expression of affection it tends to get on your nerves after a while.
Twilight Sparkle: ...you know, we're still going to need to swear occasionally.
Celestia: Swear by trees. They're fairly reliable.
Empath: The Luckiest Smurf has "getting under one's hat" as a Smurf euphemism for sex, and "the Smurfette dream" as one for Nocturnal Emission. For Wartmongers, it's "stroking one's horns" and "getting all nice and squirmy".
One Torchwood fic once referred to sperm as "cheap white carbohydrates." Made even worse when in context the author easily could have meant bed sheets, and out of context, white bread could be assumed. And ejaculate is more protein than carbs anyway.
Forbiden Fruit: The Tempation of Edward Cullen features appearances by Edward's "throbbing lavender man-fruit thing," also known as his "mancarrot", his "throbbing male object", or his "massive throbbing gigglestick." Jacob's is referred to as his "male genital item" and described as resembling a "horrible wet mushroom."
There is a fic with lines from "Jabberwocky" used as dirty talk during a BDSM sex scene. The girl (who was unfamiliar with the poem) was freaking out.
WTF Fanfiction.com has a lot of this and has even compiled a list of words fanfiction authors have used to describe genitals.