O-Ring Orifice

The human body is such a fascinating creation; its muscles, skin and tissue are able to flex, bend, and stretch to impressive lengths without permanent injury...as long as they're worked slowly, that is.

You see, unlike other materials such as rubber, human flesh doesn't have a set elasticity to it, and needs to be conditioned to stretch, bend and deform past its initial limits without risk of tearing, and even then, you risk permanently deforming them so that they can never go back to their initial stretchiness. Even if someone has a "natural" double-jointedness, he or she still must work over those joints and tendons regularly at controlled intervals to stay as limber as needed. In a porno, this is applied to the various orifices that are used during intercourse; Try to force an object too big for your "holes" without conditioning said orifices to handle it, and you risk tearing the inner walls and causing significant internal injuries.

Of course, actually looking at a porno, you'd never realize this; both the guys and the girls seem to have rubber O-rings or rubber bands in place of their actual orifices, able to stretch easily to accommodate whatever is shoved into them without injury, yet stay as tight as needed, and returning to their base elasticity and form within minutes of being "unstuffed". Oh, sure, they may scream when something exceedingly large is worked into them, but they're never really in any pain or danger, and those supposed "screams" of agony will quickly turn to cries of ecstasy after a while.

Also, if the object is not only thick but also long, there's no worry about impalement, either; the belly will just deform into the shape of the object as if the entire body were made of rubber, even shifting around so that they are literally skewered through with no problem (for some of the more extreme examples, though, this part of the trope is usually kept to the realm of fantasy and h-doujins). In reality, depending on your body makeup, anything longer than a few inches runs the risk of puncturing something, and if your belly really could deform that way, then say goodbye to your stomach muscles, which would surely split apart from the action.

In simpler terms, for all of those porn makers who use this trope, You Fail Biology Forever. The human body is a complex organism that requires constant care to avoid injury; it is not an inflatable doll.

See also: You Fail Sex Ed Forever. May be a hidden subtrope of Strong as They Need to Be.

Anime and Manga

 * The absolute master of this trope, in scale and creativity and quality is, by far, the Monzetsu series by John K. Peta, which is more or less solely about extremely elastic sex parts and the men who love them.
 * The artist Usso, known for his NSFW site named Knockout, progressed from ordinary Hentai to Large Insertion (see below, latter style). One of his characters, a green-haired girl named Izumi, slowly worked her up from ordinarily large toys to things that are thicker than her thighs.
 * In terms of absolute size Henmaru Machino is winner. His girls have unlimited capacity for every hole. And if there are not enough natural orifices, why not make more?
 * Let's not forget the uncommon hentai trope nipple fucking, which involves thrusting either a penis or sex toy into the nipples of a woman, who usually has a breast size about as believable as the act itself.
 * Which is in turn a subtrope of the Large Insertion fetish, where the girls either have extradimensional space inside them, or have very stretch-and-squash-friendly innards.
 * Often slightly subverted in that the characters do actually suffer a lot but are completely OK in the next scene. (So that the same can be done in the next episode. Some characters are just too tough for their own good.)
 * Wait wait wait, into the nipples? That's not even a violation of anatomy, it's a violation of topology.
 * Since many people don't have a- let's face it- fairly in-depth knowledge of female breast anatomy, many writers just make the assumption that a breast is a squishy balloon filled with milk, and nipples are like the mouthpiece. Of course, you'd assume that they'd be quite interested in that particular part of the anatomy, but there we are...
 * Hand-waved in some H-comics where the girl in question has undergone body-modifying surgery beforehand.
 * Cute little fairies that don't explode when humans do painful-looking things to them are often subject to this trope; this is often handwaved by saying that fairies, not being human, are a lot more flexible.
 * Subverted by a third party book of sex magic for D&D in which the mage who created the perfect magical lubricant was arrested, law enforcement having found the twisted and broken bodies of many fairies in his home. The lubricant, on the other hand, did allow for the first successful crossbreeding of halfling and ogre.
 * Similarly subverted by the hentai anime Elven Bride. The difference in genital size between humans and elves leads the male human protagonist and his titular elf wife to look for such a lubricant.
 * For that matter, subverted by the hentai Daiakuji. When title character and rather large guy Akuji marries a supposedly overage woman (who looks closer to 12), they're initially unable to...consummate, due to size differences. But solve the problem with enough conditioning. Or So I Heard.
 * A similar plot appears in another Hentai manga, Little Wife, where a newly-married girl has to, uh, condition herself so her husband wouldn't end up hurting her, by using maritial aids of gradually increasing size.
 * Mahou Shoujo Ai, also known as Sexy Magical Girl Ai, first plays with this trope, then impales it on a tentacle. While some of the girls seem to be able to endure being roughly tentacled to a terrifyingly elastic degree, some scenes then subvert it by either badly injuring or even killing the unfortunate girls.

Fan Fiction

 * Quite common in fanfiction, in which many characters may have guns, watermelons, or whatever inserted into their rectums or vaginas and everything is fine.
 * In one case, a full-sized spear, blunt end first, with the guy holding the spear telling the victim that he would take it out once it got in up to the blade. And it did.
 * Are we forgetting one Captain Jack Harkness?
 * Lampshaded, of all places, in this Digimon fanfic (Warning: NSFW); after explaining to Izzy the sex session he had with his Flamedramon (whose Biggus Dickus impaled him anally to such a literal degree that he could feel the penis' head in his throat...though, of course, with no injury(?!)), Davis gets admonished for his recklessness with a very squicky (and realistic) image of the kind of damage to his internal organs such an act should've caused.
 * There's a rather infamous piece of Fantastic Four fanart which features the Invisible Woman having sex with Mr. Fantastic (who, if you don't know, has the power to stretch). His...Mr. Fantastic Jr. can be seen practically winding its way through her digestive tract.
 * There's a fanfic for Final Fantasy VII where ..
 * There is a Sailor Moon fanfic which no one should ever read which includes a scene of insertion. And that's not even the worst part. Cue Brain Bleach.

Film

 * Used humorously in the movie Hancock: "Your head is going up his ass."

Tabletop Games

 * Unofficial "adult" supplements for Dungeons and Dragons will often include an item or spell that turns an orifice into a snug bag of holding. You're welcome.
 * A discussion of skills in a certain Image Board produced this idea: Abuse of extremely twinked-out Escapology skills allow people to squeeze through very narrow spaces. Now think about that in context with this trope. Uses of this exploit range from funny to distasteful, such as hiding the entire party inside the team tank, or diving down a target-of-assassination's open mouth to stab him from inside his gut.
 * And thanks to a Prestige Class, you can demonstrate this ability instead of making a Diplomacy check to improve an NPC's attitude.
 * On a similar, but far more disturbing, note, the notoriously awful Tabletop RPG FATAL has, among its other delights, stats for anal and vaginal circumference - stats so badly designed that it's actually possible to have a negative anal circumference. That probably will tell you most of what you would (not) want to know about the game.

Video Games

 * Non porn example (mostly, anyway) in the Nasuverse any of the female protagonists that haven't been raped repeatedly are virgins, yet they still seem to have one of these. After a minute or two of pain they're generally okay and even ready to go again. As an aversion Tohsaka is in pain throughout the entire encounter because not only does she not have one, she's also not exactly in the mood. Since they've grown more popular they try and tone the sex down in adaptations and may do without it entirely in future works.
 * The image shown above is a running joke about one of the Spy's lines from Team Fortress 2 and his constantly smoking and flicking of cigarettes. This troper just wanted to comment because (although he can't prove it), he vectorized the original image that the meme sprung forth from.

Web Original

 * What, why shouldn't I go- AHH! AHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH!
 * On a related note, the horror  large objects... Even if the object
 * Oh, c'mon. It can't be that ba--OH GOD! OH GOD! OH GOD! AAAAAAAH! AAAAAAAAAAAAAAAAAAAAAAH! AAAAAAAAAAAAH
 * I know I was told not to Google it, but it can't be as bad as some of the stuff I've seen. Let's see what all the fuss is ab- AAAAAAAAAAAAAAAHHHHHHHHHHHHHHHHHHH!!! This Troper is now blind, crying, and screaming in between fits of vomiting.
 * AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAHHHHHHHHHHHHHH
 * The phrase "nightmarishly elastic" is quite apt here.
 * If you liked, you may enjoy the.
 * Two more words:
 * Somewhere on the internet is  a woman.
 * Unless we're talking about different clips, I'm pretty sure that's fake. Or at least really hoping it is, for her sake...
 * That sounds like in Sonichu.
 * The picture of the guy.
 * The artist Mamabliss does a lot of art in numerous categories that features all sorts of impressive stretching on the part of the girls, especially when there's unbirthing involved. For the curious, unbirthing is exactly what it implies - climbing back inside the girl's womb.
 * The picture of the guy.
 * The artist Mamabliss does a lot of art in numerous categories that features all sorts of impressive stretching on the part of the girls, especially when there's unbirthing involved. For the curious, unbirthing is exactly what it implies - climbing back inside the girl's womb.

Western Animation
"Homer/Paul Bunyan: "Y'know, we've been together a long time now. When are we gonna ... you know?" Marge: "Soon, soon. I just need a few more yoga lessons.""
 * On an episode of The Simpsons where Homer was cast as the giant Paul Bunyan in a hobo's story-telling: