A Stargate Atlantis fic came up with "ice farmer" as an Alien Euphemism for "gay," leading to a particularly wonderful example of Metaphorgotten. ("If the ice wanted to be farmed...")
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.
A 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.
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.
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!"
Forbiden Fruit The Tempation Of Edward Cullen features appearances by Edward's "throbbing lavender man-fruit thing," also known as his "mancarrot" or his "massive throbbing gigglestick." Jacob's is referred to as his "male genital item" and described as resembling a "horrible wet mushroom."
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.
A Kim Possible fic mentions that: Her (Warmonga's)"quarlaps" were getting very warm.
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.
"Marry My Father Please" features a subplot in which Anko's birth control has failed spectactularly, yet she insists that she only has the flu. And... well... see the quotes page.
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.
Fever Dreams: when Light tells L "I like it when you investigate me."
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".
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.
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.
One Teen Titansfic uses the expression "The deed of a thousand euphemisms". Judging from the rest of the examples here, it is more than justified.
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."
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.