In works involving non-humans (whether it be anthropomorphic beings, mythological creatures, or aliens) having sex, there's a tendency for everything to still use a human-style sexual system (most visible in the penises). Everything will have sex organs based around humans of a size that allows the owners to copulate with each other as the plot demands. Often, this manifests itself as a differently-colored human penis/vagina, but even when some design consideration is given towards making them look more like the species the creature is based on, they still have a tendency towards human proportions. And human stimulationit's believed that primates and dolphins are the only creatures that copulate when not fertile.
In Real Life, the genitalia of even closely related species often have significant differences, making cross-species coupling impossible (except for bestiality in some cases), even if they both follow the "Sword and Sheath" model.
- Played completely straight with Humans and Zentraedi in Macross (and by extension, Robotech's adapatation of Super Dimension Fortress Macross): the two species can have children without any sort of outside aid, as seen when Max and Milia have a daughter shortly after getting married. It's justified in the Macross continuity by the fact that the two races are almost genetically identical due to both being created by the Protoculture, with Human-Zentraedi hybrids having become a common sight in the later series. In fact, all the Protoculture-influenced humanoid species seem to be compatible with each other; for example, depending on what source you're looking at, Mikhail might actually be part-Zolan (who are a marsupial species)note .
- Ironwood uses this trope, although there it is mostly a case of the female genitalia of the fantasy races being compatible with human male equipment. The males occasionally have trouble finding the right spot but, once they do, there are no problems doing the deed.
- Tank Girl routinely gets it on with mutant/genetically engineered kangaroo Booga. His... equipment... is human. Probably for the best. Kangaroos have Exotic Equipment.
- XXXenophile switches between this and Exotic Equipment.
- Subverted, or at least downplayed heavily, in The Next Frontier. Despite the fact that in this particular 'verse the Kerbals have radically different reproductive biologynote from the aliens whose satellite TV they're watching, Jebediah enthusiastically declares that "a certain amount of creativity might be required, but I don't see any insurmountable compatibility issues so far" for purely recreational sex. It's immediately suggested that he hasn't been laid in far, far too long.
- A number of Guardians of the Galaxy fanfics invert this specifically to invoke Boldly Coming; this trope applies for almost all alien species except humans, and Peter gets laid so often by picking up adventurous women interested in Exotic Equipment.
- District 9 mentioned, but didn't show, "interspecies prostitution". One wonders as to the mechanics, as the Prawn are naked for the whole movie and don't have visible external genitalia. They are shown to reproduce by laying eggs. One assumes, given how grossed out everyone is by the idea, that it is just about as natural and comfortable as having sex with a chicken (which people actually have been known to do). In other words, it's taboo even amongst deviants but there's some people who are just too damned kinky and/or curious not to try. Alternately, it's just propaganda against the Prawns and, to a lesser degree, the lower class. Given the film's themes of racism—against both the Prawns and other humans—this interpretation wouldn't be out of place.
- In Galaxy Quest The human-looking aliens are actually some sort of squid thing under their holograms. Chen and Laliari don't seem to care.
Guy: That ain't right!!!
- Lampshaded in Star Trek VI: The Undiscovered Country when Kirk fights an alien in the penal colony. Lying on his back, he kicks the alien in a jointed area in the middle of his legs. The alien immediately keels over groaning. When McCoy says "It's a good thing he had knees," Martia, the Chameloid, says, "That was not his knee. Not everyone carries their genitals in the same place." Cue shocked look on Kirk and McCoy's faces.
- In The Shape of Water, Elisa uses some interesting hand gestures to explain to an increasingly astonished Zelda why the Mermaid Problem was not actually a problem.
- SF author John Varley absolutely loves this trope:
- The Gaea Trilogy has the Titanides, an alien species who are sexually compatable with humans because they were deliberately engineered to be sexually compatible with humans.
- Steel Beach has the ULTRA tingle system, which gets around the whole genital compatibility issue entirely by using wireless modems for sex.
- The actor protagonist of The Golden Globe equipped himself with a penis that can be turned inside out for use as a vagina, primarily because it doubles his chances of getting roles.
- Finally, the novel The Ophiuchi Hotline has literal male to female universal adaptors that allow characters who go in for radically modified genitals to "dally" with their more conservative counterparts.
- In The California Voodoo Game, two conspirators meet at a sleazy hotel called the Mate & Switch. Each room is equipped with sensory-simulation bodysuits that let the wearer experience the sensations of being the opposite gender, making each a literal Male-to-Female Universal Adaptor.
- Simply glossing over this (very Edwardian, of course) doesn't mean it's not there; John Carter of Mars and his red Martian princess Dejah Thoris. Would not her biological equipment for laying eggs cause at least a bit of a shock for our Earthly hero?
- An interesting version occurs in the Super Hero novel Soon I Will Be Invincible. The heroine Damsel is the daughter of a human male and a Green-Skinned Space Babe, however, she was bred in a laboratory because her parents were not compatible with each other. Her mother wasn't even a mammal.
- Played with in James Patrick Kelly's Look into the Sun: The protagonist is sent on an interstellar trade mission that requires him to be transformed into an alien species that just happens to use sex as a bonding and bargaining tool. Fortunately, he's also been equipped with a neural AI that can translate all of the alien physical sensations into the human ones he's familiar with... until the AI gets pettish mid-screw and refuses to translate any further, leaving him to cope with his new Bizarre Alien Biology on his own, such as the sudden realization that his delicate, flower-like genitalia means the females have been taking the dominant role during copulation all along.
- In West of Eden, Yilanè have reptilian genitalia, including hemipenes on the males (who also function as Mister Seahorse). However, the female genitals are compatible enough with human equipment that Vaintè and Kerrick are capable of having plenty of sex while the latter is the former's personal slave. It should be noted, however, that Vaintè is insane both by human standards and those of her own race, and Kerrick was raised by Yilanè and has (at this point) an incomplete and screwed-up understanding of human sexuality. When Kerrick returns to humanity, he starts looking back on his time with Vaintè in disgust and horror.
- Alan Dean Foster:
- In Spellsinger it looks like all sentient mammals including humans are compatible, if not able to crossbreed. Jon-Tom decides not to investigate any further, when he gets a chance.
- In Quozl, the eponymous aliens are human-sized marsupial rabbits, who copulate for recreation 20-60 times a day.note For some reason their scientists are shocked to learn they are compatible with humans and decide to keep it a secret from the rest of the population. (Preventing humans from going Out with a Bang probably wasn't the reason. And it didn't happen when the truth was revealed. More likely it was done to prevent Quozl from experimenting and to stay hidden.)
- Subverted in A Door into Ocean. The Sharers of Shora are all-female lesbian fish-women who, after centuries of Asexual Reproduction, are unable to have penis-in-vagina sex. Mentions are made of Valedonians using rape to torture Shorans.
- Isard's Revenge: This is discussed in regards to Gavin's relationship with Asyr. While he says "the parts line up just fine" it seems they can't have children. Not surprising, given that Bothans appear to have evolved from canines.
- In the Babylon 5 spinoff Crusade, Captain Gideon accidentally acquires a data crystal containing Pak'ma'ra/Human porn. The ship's doctor walks in and comments that this shouldn't work (the Pak'ma'ra are humanoid, but closer to Starfish Aliens than Rubber-Forehead Aliens with tentacles around the mouth and reproduce asexually). Then they bend their necks ninety degrees with a look of horror on their faces. Gideon remarks that technology is a wonderful thing.
- Babylon 5:
- The series unexpectedly describes Centauri genitalia quite explicitly and as being mostly (if not entirely, barring near-perverse levels of creativity) incompatible with comparable human anatomy. Although looking like they should be just simple Rubber-Forehead Aliens (they look like sharp-toothed humans with strange hairdos), genetically they have nothing in common with us. To the point they have 6 tentacles (which are their penises) on the male and 6 vaginas on the female's backs. Their equivalent of kissing is penetration with just 1. Oh and the tentacles are prehensile and Londo gets caught using one to cheat at poker...
- G'Kar (before his spiritual transformation) ( and after: remember the removable eye on the wedding night) is seen as being also a bit of a pervert, being into Human women as well as at least one Centauri female. His species, although looking reptilian, are marsupials, including keeping their young in a pouch. As Na'Toth said in her first appearance, "It is not my place to speculate on how anything finds its way into your bed..."
- He also propositions to Lyta, but only in order to re-introduce the telepathic trait into the Narn gene pool, implying that not only is it possible for a human and a Narn to have sex, but that viable offspring may result from it.
- A throwaway line about "what are these cramps I'm getting" when Delenn becomes a half-human hybrid implies that her species, at least, has a rather different reproductive system than humans. On the other hand, that could just mean that her species' females don't menstruate... which is true of most mammals here on Earth.
- Most alien species, at least those encountered by Starfleet, seem to be sexually compatible with humans in the Star Trek universe. Even those that shouldn't be, like the Cardassians (which, according to canon, supposedly evolved from reptile-like ancestors, but are capable of mating and reproducing with mammalian humanoids). In an unexpected subversion of this, it is noted that Klingons, Vulcans and humans, while sexually compatible, have a difficult time producing hybrid offspring "without help". In the Next Generation episode "The Chase" it is revealed long before humans, Klingons, Vulcans, Cardassians, and other races came into being, there was a dying race who spread their genetic material to developing worlds in hopes of their legacy continuing. This is likely the reason why there is such genetic compatibility.
Kim: I've got to be honest, I wasn't expecting something so different.
- Averted in the Star Trek: Voyager episode "The Disease", when Ensign Harry Kim has a passionate affair with beautiful humanoid alien Tal, then has the following Getting Crap Past the Radar discussion.
Tal: Neither was I. Our species look so similar... Well, at least on the surface.
Kim: I would've never guessed when it came down to the basics. Well, let's just say the birds and bees will be very confused.
- Subverted in the original miniseries V (1983), in which a reptilian humanoid is required to undergo surgical alteration (and make no mistake, even the simple act of implying as much was pretty racy for primetime in 1982!) before mating with a human female. The novelization goes a little further than the TV show did, noting that the human subject of the experiment — although unaware of how alien her lover is — senses that something is wrong.
- In V (2009), it is perfectly possible for a Visitor to have sex with a human, as evidenced by two of the show's interspecies couples. One of them, in fact, results in a Half-Human Hybrid, something the aliens thought impossible given the two race's diverse origins.
- Subverted in Lexx with Stanley and Lyekka. Although they would have sex if given the chance, Lyekka is a highly evolved plant and has no genitalia. There's also the issue of Lyekka's entire raison d'etre being to seduce other species by appearing in a form desirable to them for the purpose of EATING THEM. Despite her affections for Stanley, she is still a plant that might just eat him if they ever tried doing it.
- Hilariously conversed about in The Big Bang Theory: After the gang speculate on how Amanda and Sarek conceived Spock, considering their difference in species, Raj muses on how in Star Trek everybody's genitals seem to be the same: "No alien lady ever told Captain Kirk 'Hey, get your thing out of my nose!'"
- Presumably the case in Farscape, at least most of the time, though you never actually see the genitals. Implied by Chiana and D'Argo, for instance, given that their sexual positions seem very human-esque and even though they are very different species explicitly stated as not genetically compatible, they still manage to have lots and lots of sex.
- D'Argo (a Luxan) had a hybrid son (Jothee) with his Sebacean (augmented human) wife. Jothee also has lots of human-like sex with Chiana. Zhann (a blue humanoid plant) at one point states that Luxans 'make excellent lovers', implying the two can have sex, but not necessarily reproduce.
- Aeryn (a Sebacean) has a child with Crighton (a human), even though she details many large differences in her reproductive biology.
- Sebaceans do originally come from basic human stock, but were genetically altered by the Eidelons in order to serve as their Peacekeepers.
- Apparently, Sebaceans are also technically genetically compatible with Scarrans, who are, more or less Lizard Folk (although their females look more human). Scorpius is the result of a female Sebacean being raped by a male Scarran (in fact, he was at first told it was a male Sebacean who raped his female Scarran mother, while in truth he was the only viable offspring produced by Scarran research into genetic compatibility between the species). He does have many health issues and can only survive with cooling rods inserted directly into his brain because his Scarran physiology produces tons of heat which his Sebacean physiology is especially vulnerable to and unable to deal with.
- Exalted averted this trope with an April Fools sex supplement. One of the artifacts in the PDF is called the Many-Stalked Flower of Delights, which allows beings of incredibly different physiology to enjoy the pleasures of each other's company. The creator of the artifact was an Eclipse who apparently fell in love with a mountain. Not the god of the mountain, the mountain itself. The first use of the item caused a volcanic eruption. Of course, the Many-Stalked Flower of Delights is a Male-to-Female Universal Adaptor, but it's the opposite of this trope.
- Apparently in Mass Effect human and Quarian genitals are similar enough that sex can occur normally once you get around the problems inherent in cross-species fluid contact.
- Don't forget the other races; heck, Mordin has pamphlets on cross-species intercourse for whatever Romance Sidequest you pursue. Drell, Turian, Quarian, he's got you covered. It's not made clear if all the parts line up precisely. The pamphlets are necessary to achieve proper positioning and to hit the right erogenous zones. All we really know is that the respective parts are in the same general area on the humanoid species. It's also for health safety. Quarians with their atrophied immune system are only an extreme case. There are also all kinds of allergies to the body chemistries of other species. There can also be mechanical issues. One pairing (Female Shepard and Garrus, a Turian) has chafing as a specific risk.note
- Asari do not appear to have this problem, mostly because nothing needs to be, ahem, inserted. All you need is a nervous system and they can initiate melding at a touch, although this doesn't stop them from engaging in physical intercourse. Lampshaded in Mass Effect: Andromeda, where Peebee (asari) and Jaal (angaran) are both curious as to whether asari reproduction would still work with the angara, despite them having evolved in entirely separate galaxies. Though they are not prepared to find out personally with each other.
- Celeste Luvendass, a Secret Character from HuniePop; Kyu even lampshades it.
- In Drowtales, because dragons are descended from elves who were mutated or changed into the four legged lumbering beasts they are today, they are able to mate withnote and produce offspring with normal drow.
- Zhor, a giant purple spider is able to mate with and knock up Mel'arnach to produce Ariel.
- The webcomic Dreamwalk Journal does this with human-insect/arachnid hybrids, although everything seems to be scaled up quite a bit, even for visiting humans.
- Justified in that, according to Word of God, the hybrids and their sexual customs were all created by human genetic engineers centuries or millennia in the past.
- Averted in The Order of the Stick with lizardfolks, who have a genital sheath and hemipenes, like most reptiles. Though this doesn't seem to stop interspecies prostitution, at least with the females.
- Undead seem to be able to breed in both Grim Tales from Down Below and Dungeons & Denizens; detailed explanations on what male "biophiliacs" (including skeletons with no flesh) do for sex and procreation:  . So while it goes unmentioned, they MUST use some adaptor, right? Yes. It's called another dude, and an abortion clinic. Think about that for a minute — how else did you think a skeleton would have a dead baby?
- Jay Naylor varies use of this trope. Sometimes male organs are human-like, other times (mostly with equines and canines) they are anatomically close to the Real Life animals. Females, however, universally have human-like vaginas.
- Hilariously averted in Harry Partridge 's short "Hot Na'vi Sex". Parodying the romance sub-plot in Avatar, Neytiri and Jake (in Na'vi form) are shown just about to have sex. However, Jake is surprised when he is told to use his "ponytail". When Neytiri doesn't seem to understand what a "penis" is, Jake goes on to pull out what turns out to be a "Soup Drinker", much to Neytiri's annoyance (we even get a full view of it, which is just a bendy-straw like thing coming out of Na'vi!Jake's crotch). Likewise, female Na'vi seem to have an orifice used to play wind instruments where their vaginas should be. Na'vi sex turns out to be, logically enough, intertwining their ponytails.
- Neytiri: JAKE! Is this really the time to take out your soup drinker? We're not even drinking soup right now!
- The title trio of the American animated series Aqua Teen Hunger Force have all displayed sexual interest in and claim to have had sex with human females at one point or another, despite the apparent lack of genitalia (and the fact that all three are a giant, anthropomorphic foods — a box of fries, a milkshake and a meatball). And yet two have vaguely phallic appendages and the third can change his form a bit... However, it should be noted that Shake is a huge goddamn liar, and Frylock mentions at several points that Shake doesn't have a penis. When Shake claims to have had sex one time, Frylock's response is "When? How? With what?" It's repeatedly mentioned that Frylock is a virgin, but this seems to be attributed more to his nerdiness than any physiological shortcomings. Finally, any claims Meatwad makes are probably made without any understanding of what he's talking about, just parroting things he overheard from Shake, Frylock, or Carl.
- In Ben 10: Alien Force, the main source of superpowers is revealed to be having alien ancestry, which includes, among other things, energy beings (although granted said energy being could assume human form) and an alien made out of living fire (which is a bit harder to explain). Plus there's the whole human marrying Mud Monster story from the previous series....
- It's implied by the amount of cross-species bonezoning that's implied or outright stated to go down in Rick and Morty that this is the rule, not the exception. At the very least, we know that Bird Person, an alien, has a penis and that Tammy (one of Summer's Earth friends) really likes it. Tammy turns out to be an alien in disguise, but not one from the same species as Bird Person, so it still counts.