Thanks to inherent biological traits, some form of Applied Phlebotinum, etc, two characters are in a situation where they have to have sex in order to save their lives. Frequently used in Fan Fic, especially Slash Fic.
A variety of Deus Sex Machina, with a bit of Intimate Healing thrown in. Compare with Aliens Made Them Do It. If saving the species is the reason then it might be an Adam and Eve Plot.
If the characters merely "need" sexual satisfaction to improve their mood and disposition, it's an example of You Need to Get Laid — this trope is for situations in which they literally do need to have sex.
There is a case of Truth in Television with this trope: Female ferrets can actually die from anemia if they don't mate when they go into heat.
The inverse of this is Death by Sex.
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Anime and Manga
In a yaoi called Animal X a race of reptile like people need to breed to save their race, the only problem is there are zero females and they don't want to mix blood with humans, so when one of them does find a guy who turns out was a girl at birth (even though no one told him) and he has their blood, well the whole thing turns into this guy who didn't even know he was a girl being nearly raped(eaten in one occasion where the guy was hungry and forgot he what he was suppose to do) by every other guy he encounters, plus he is slowly turning back into a girl.
Tristan: He wants a virgin sacrifice! Quick, Téa, have sex with me! It's the only way to stop him!
Téa: Hell no, I'm saving myself for Yuugi! ...I mean marriage.
Ninja Scroll gives us Jubei, poisoned and seeking the antidote. As it turns out, the only way to stop the poison is with more poison. Good thing his partner is a sexy ninja with a Kiss of Death.
Interesting subversion; She offers herself — naked — to him. He refuses the sex, seemingly on principle. Insult to injury, before she dies, she confesses she loved him,and wished they could have made love before she died.
ElfQuest has Recognition, a telepathic mating urge that forces elf couples with good genetic matches to mate and have children. Elves who try to resist become sick, though Word of God is that the elves won't actually die, since that defeats the point. They may feel like they're going to, though.
It should be noted that (1) these elves are almost always overjoyed with the opportunity to have a child, and (2) there is no such thing as cheating in the ElfQuest world, and more often than not Recognized mates are instantly welcomed into their new family, forming bisexual three-way relationships. Only two cases of elves being unwilling to mate are at all relevant to the plot: the main characters Cutter and Leetah, who are both just being stubborn and end up Happily Married; and the young wolf-elf Dewshine with the old bird-elf Tyldak, because Tyldak was a villain at that point. (Ironically, Recognition allowed them to see each other's "true selves", which caused Dewshine to be somewhat attracted to Tyldak - seeing his original body, not his flesh-shaped one - and Tyldak to be absolutely repulsed by Dewshine - seeing her wolf heritage instead of her regular pretty elf face.)
Also a case of Mother Nature Knows Best. The result of a Recognition-forced mating will always (1) successfully come to term as a healthy baby (unless an outside force prevents it), and (2) be a genetically superior child who has all of the parents' genetic advantages and few, or any, of their weaknesses. Often, the offspring will manifest entirely new abilities or powers, as well. E.g., if one of the parents is a genius who possesses extremely high psychic ability at the expense of physical frailty, while the other is a half-witted, psychically inept world-class bruiser, the resulting child is pretty much guaranteed to have similar psychic ability to that of the first parent (if not more), tremendous physical strength and endurance, and a genius IQ. The drawbacks will simply never manifest.
Also, in the New Blood series, Dodia and Door were Recognized, but the ex-Blue Mountain elf soon crossed the Moral Event Horizon as a villain, such that she had to bash him over the head with a club to end his crimes. Whether or not she could've done this before they'd had sex is unclear.note Dodia is shown as having gone completely bonkers later on
In the Harry Potter fandom, the Snape/Lupin ship saw a great number of fanfics using this trope, where Snape and Lupin were imprisoned together right before a full moon. This would mean Snape's death as soon as Lupin took upon his werewolf form, but if Lupin considered Snape as his "mate", the werewolf would spare them. Therefore...
A particularly horrible fanfic featured a once-in-1000-years event where Hogwarts ghosts would compulsively kill anyone who was innocent (i.e. a virgin). Hermione didn't quite get out, so Snape had to go in to rescue her... yes, that way.
It was also a particular favorite of Harry/Draco writers, who invented the Male Veela fic, where poor Draco had to bond (sexually, of course) with his Soul Mate, or perish. Naturally, his worst enemy was his destined Soul Mate.
The fandom has an Epileptic Forest involving the sex lives of werewolves, with (mostly Slash Fic) writers coming up with new ways of justifying Lupin shagging anyone in the cast.
In Star Trek fandom, Vulcans go through something similar called "Pon Farr." The very origin of Slash Fics comes from corny situations where Spock had to mate with someone (Kirk) "or else." The various shows themselves found loopholes to get out of the implications.
It was 'Mate or Die' originally, but turned into 'Mate, Fight or Die' to give the writers an out in the ST: Voyager episode 'Blood Fever'. The idea in TOS 'Amok Time' was that if anyone challenged a Vulcan's right to his wife while he was in the throes of the blood fever they'd literally have to kill him to stop him. Later writers missed the point (perhaps intentionally) that Spock was snapped out of Pon Farr by the shock of thinking he'd killed his friend. His line "It must have been the combat" was a cover for this (embarrassing) emotional reaction.
It's also notable that originally, pon farr was something that only Vulcan males were subjected to. By the time of Star Trek: Enterprise and later Expanded Universe literature, pon farr became flanderized to where it affected all Vulcans.
"This is a fanon invention that has Elves, who are dying either of wounds or grief, "bond" with another Elf or man, by mingling their blood, having sex and thus getting new strength; sort of leeching life off the other and forever living like that. In this case, if one of the Elves "bonded" dies then the remaining Elf must find someone else to bond with or also die. I found no explicit or implicit grounds for this in Tolkien's writing. Still a very interesting concept. Once again Laws and Customs of the Eldar, History of Middle-earth: Morgoth's Ring holds the most information regarding elven relationships."
A Darker and EdgierRedwall fanfic used this to explain Veil's conception. Swartt had apparently no interest in Bluefen in canon, yet they had a kid. The characters involved are anthropomorphic ferrets, and the females have a tendency to develop diseases of the bone marrow if they don't mate at the appropriate time. Sooo ...
there's a Doctor Who fanfic, A Feline's Fancy with a subplot where a female human pet of the Catkind wants the Doctor to get her pregnant so the Venemor, a race of aliens with spiky heads, can't get her pregnant with their spiky-headed babies, which would kill the slave girl in childbirth (yeah, a definite ouch and Squick). The Doctor doesn't go through it, but saves her later from dying in childbirth.
Another, rather less thought-through Doctor Who fanfic unintentionally managed to point out the Unfortunate Implications inherent in the Mate or Freeze scenario: The Doctor and Rose are trapped somewhere and Rose is freezing to death. Since the Doctor's natural body temperature is too low to do Rose any good, he grudgingly resolves to do the one thing he can to raise his own temperature, in order to act as a thermal blanket for Rose afterwards. However, the writer has him angst about this decision for a while, because Rose is near catatonic with cold at this point and really too out of it to consent or get any pleasure out of the act. He even worries that her body has shut down too far to make sex physically possible. But then he finds that she's still wet enough that it won't hurt her, so it's all good. Except that the readers are left to ask themselves why the Doctor had to involve Rose's anatomy at all, if all he needed was to induce his own physical arousal...
There is even a series of oneshots on Adultfanfiction where the Doctor crosses into Star Trek universe and gets 'infected' by Vulcanian pon farr. This makes for one happy Jack Harkness, companion at the time. After the first, oneshots gets weirder and weirder and could be squicky to someone.
Neon Genesis Evangelion: There's an fanfic Bound. where Shinji and Asuka are kidnapped by SEELE and put into a meat locker naked as punishment for Shinji aborting Third Impact. Naturally, Asuka comes up with an idea how they could make more heat before the dropping temperature kills them. Cue this trope.
Naruto: There is an Ita Saku oneshot floating around in which Sakura had ingested nightshade, and the fastest way for her to burn it out of her system long enough to make an antidote is to have sex with a certain Uchiha.
This trope comes up fairly often in any given fandom. Often takes the form of, essentially, something akin to this: a character afflicted with a poison/drug/substance/aphrodisiac/etc that for whatever reason amps up their sex drive times eleven. Not only does it make them virtually crazy with lust, their partner is confronted with the knowledge that if the afflicted is not satiated they'll suffer horrible consequences ranging from extreme discomfort/injury, to insanity, to actual death. Mate or die, truly. Oh and having sex once isn't good enough. It's going to last for hours.
A milder version comes up in quite a lot of My Little Pony: Friendship Is Magic lemons: Ponies have a mating season just like real horses (or as similar as the author's knowledge of horses allow). While they don't die if they don't have sex, they become quite obsessed with it and it makes anything but trying to get laid very hard, justifying OOC behaviour and the like. The end results vary: In Why Didn't Ah Stay At The Farm, Big Macintosh makes the mistake of going into town during that time, while in Fun Bus to Manehattan the Mane 6 decides to counter the lack of eligible males in Ponville by just taking the bus to Manehattan for some quick sex tourism (NSFW, though the mood of the stories for each of the Mane 6 is actually quite different).
ThisAngel fanfic, "Offerings", uses this plot. The author's summary is simply "Demons made them do it", though the characters were in an established relationship.
One As the World Turns fanfic has has Luke and Reid snowed in and have to get warm. It turns out Reid actually had that planned all along. Reid's a doctor, so Luke believed the having to keep each other warm excuse.
Stargate fandom has an entire sub-genre often referred to as "Aliens Made Them Do It". Generally, two or more characters are forced to have sex in order to conform to some bizarre alien ritual. Examples range from humorous crack fic to extreme pathos.
Once Bitten: In the (very early) Jim Carrey comedy, the ancient vampire played by Lauren Hutton has to feed on the blood of teenage male virgins to renew her youth. Carrey's girlfriend saves him before the third (final) bite. No, not by killing the vampire, smart guy.
Film example, Stephen Chow movie Royal Tramp 2. A villainness is hit with a poison that will kill her before dawn unless she mates with a man. The lucky man turns out to be, you guessed it, Stephen Chow.
Actually, she was the villainess for the first movie and about half of the sequel, until the aforementioned scene took place. After that, she's basically part of Stephen Chow's harem.
The Invention of Lying: Invoked when Ricky Gervais' character discovers lying for the first time in human history. To test his new-found ability, he stops a random attractive woman on the street (Stephanie March) and tells her that unless they have sex "right now" that the world will come to an end. Never realizing that he might not be telling the truth, she agrees in an instant.
"Do we have time to get a hotel room?"
There is a certain irony in the fact that Ms. March played the ADA on Law and Order: SVU for a number of years.
In History of the World Part I Mel Brooks' character of King Louis during the French Revolution offers this choice to an attractive young courtesan. If she doesn't have sex with him, he will kill her father as a political prisoner.
King Louis: It's up to you: hump or death? Come on, tick, tock. Hump, death, hump, death, hump, death ... Courtesan: Okay, okay! Hump!
The book The Mote In God's Eye by Larry Niven and Jerry Pournelle invokes this without the Applied Phlebotinum. The plot of the novel involves First Contact with a species of aliens who literally die if they don't mate and get pregnant due to their Bizarre Alien Biology. The resulting overpopulation hasn't been very good for their long-term prospects as a civilization.
In the novel Innocence by Jane Mendelsohn, the main character believes that her stepmother will kill her to drink her virgin menstrual blood. She conveniently has a boyfriend, so she cures herself of this virginity thing, much to the anger of the stepmother, who really was after her blood.
Kathleen Morgan's narmy romance novel Heart's Surrender featured a race of vaguely felineHuman Aliens...who were at very least unlikely to survive the night if they didn't have sex when their planet's three moons were aligned.
Used in the Dragonlance short story Raistlin's Daughter, in which a mysterious woman accidentally forms a painful magical connection with the titular character that can only be severed if she bears his child. When she finally tracks him down and gets him alone so she can explain, Raistlin is less than pleased. Of course, they're conveniently alone in a cave in the middle of a blizzard with no reliable source of heat. note Three guesses as to who lured Cameron away. And wiped both Raistlin and Cameron's minds afterwards. Though she claims that is was because if they knew her race, they'd hate her and the child.
Played straight in Sherman Alexie's short story The Sin Eaters, where the narrator is twelve years old and all the American Indians are rounded up and placed in facilities where they are forced to donate blood and marrow. This is because American Indians have an immune gene of some kind that protects against a virus, or something. They talk about antibodies, and that's all I know. Anyway, the government forces him to breed with a thirty-year-old woman who has already been with several men because the more men she's with, the stronger the child will be. That last one, despite failing biology forever, is actually congruent with the myths of some Native American peoples.
The Jennifer Morgue by Charles Stross has Ramona Random, an assassin from the CIA's occult equivalent with a succubus bound to her soul. The thing is regularly "fed" by consuming the souls of people Ramona has sex with... and when it gets hungry and Ramona's mark dies of a heart attack before climaxing, she's in a rough spot, as "if it doesn't get the little death, it'll go straight for the big one."
In Women Of The Otherworld, Karl tries desperately to stay away from Hope, but ultimately is unable to because his inner wolf has chosen her as his mate and he can't bear to be away from her any longer. It gets to the point where he practically jumps her in her apartment (though he does let her know that at any point she can tell him to gtfo and he'll comply).
Disturbingly, the next book in the series introduces the kumpania, a cult of inbred clairvoyants who are required by their own laws to produce a child within a year of marriage. Any who fail to comply go through what can easily be called group rape - the woman is forced to have sex with all the other adult males in the kumpania until she gets pregnant.
Jory and Kol in S.L. Viehl's Blade Dancer.
Lisanne Norman's Sholan Slliance series features Leska pairs who live and die by this trope.
Live Action TV
One of the most common examples is, of course, the "we're trapped somewhere really cold" situation, where there's only one logical way to get warm . . . This is partially Truth in Television, because the most effective way to warm up a hypothermia patient in the wilderness is to share a sleeping bag with them, but both the warmer and warmee can keep their underwear on.
That exact fact was used as an innuendo in The X-Files episode Detour. Scully tries to keep Mulder warm after he goes into shock from being attacked by a wild animal. Mulder quips "I've heard the best way to generate body heat is to crawl naked into a sleeping bag with someone else who's already naked." Scully replies "Well, maybe if it rains sleeping bags, you'll get lucky." Unfortunately, this never happens.
One example of this was an episode of Taxi, with Latka Gravis and some random female cabdriver. They do it, and it almost destroys Latka's marriage.
Well, technically it does destroy his marriage. They then remarry 30 seconds later swearing to "really try to make it work this time"
Played for laughs in Boy Meets World when Eric and a rather attractive female co-worker with a good deal of out door survival knowledge get trapped in their car. She does this once Eric begins to sucumb to the cold (off screen of course, as this is Disney) and explains how they survived later. Eric is mortified that she didn't wake him so he could enjoy being under the cover of a sleeping bag naked with a girl. She does seriously point out that they were in a little more danger than casual sex would permit.
This is — no, really— canon in Skins, with bonus incest: in retaliation for a vicious prank, Josh Stock kidnapps Tony's sister Effy, overdoses her with heroin, and tries to force Tony to have sex with her before he'll call an ambulance, although he inexplicably (very likely for real-life reasons having to do with Kaya Scodelario's age) backs down rather than make them go through with it.
The writers of Red Dwarf wrote a script "Identity Within", where the Cat has to breed with a female of his species or be killed by a growing poison sac in his body (a reading of the script by Chris Barrie is available on the Series VII DVD). Unfortunately it proved too expensive to film, which means (ironically) that Red Dwarf's resident ladies man is the only member of the crew who never gets laid.
In Star Trek: The Original Series in the episode "Amok Time" Spock must return to Vulcan and mate or he will die. Apparently all Vulcans go through this process (known as Pon Farr) every 7 years. The situation is subverted when Spock believes he has killed Kirk and finds he's no longer in the mood. (This episode probably launched more Slash Fic than you can even imagine.)
Try just "launchedSlash Fic in general". Hmm, Spock suddenly didn't need teh sexxorz after he thought he'd killed his bestfriend? Gee, I wonder what the implications of that are?
On Star Trek: Voyager they solved Tuvok's pon farr with the holodeck, though he was interrupted the first time.
Another Voyager episode solved Tuvoks pon farr by having the Doctor SING at him until they could knock him out. Subversion, since it was just a holodeck simulation by the Doctor.
On yet another Voyager episode, Ensign Vorik, a Vulcan, mind melds with Torres while in the throes of Pon Farr, thus "infecting" her with it. Paris agrees to do the deed to cure her, but Vorik is infatuated with Torres and challenges Paris to a ritual fight for her. Torres is having none of that, so she decides to fight as Paris' champion. She and Vorik end up fighting each other, thus purging their Pon Farrs without sex.
And in Star Trek: Enterprise, an alien virus caused T'Pol's Pon Farr to manifest early. A cure was found before she could convince someone to do the deed. In the Mirror Universe, however, Tucker does make reference to "helping" her with that problem.
In an episode of Tracker, one of the aliens has a build-up of poisons that must be "released" daily, in order for him to survive. To that end, he becomes a pickup artist, sleeping with women and leaving before they lapse into a coma. Needless to say, he gets very upset when he is just about to do it with yet another woman, and she suddenly changes her mind, as he doesn't have time to go look for another.
A variation in Farscape. If the princess doesn't marry John, she can't be queen because he's the only male who can give her healthy children (a prerequisite for becoming queen); if she can't be queen, the crown goes to her brother; if he becomes King, he allies the Kingdom with the Scarran Empire, the Peacekeepers invade by way of retaliation and hundreds of billions of people die- likely including the princess. On the other hand, if John doesn't marry the princess, he gets turned over to Scorpius to be tortured and killed.
A variation in Doctor Who. In "The Doctor's Daughter", the Doctor inadvertently creates a fully-grown Opposite-Sex Clone, after being forced at gunpoint to put his hand into a DNA scanner.
This trope is the basis for an old joke: A doctor and his residents are making the rounds when they discover a man masturbating in one of the examination rooms. One of the residents asks why he is doing this, and the doctor explains: "This man has a rare condition where he must have an orgasm every hour or he will suffer a fatal hemorrhage." Several rooms later, they come across a man receiving a blowjob from a gorgeous blonde nurse. Again the residents ask what's happening and the doctor responds: "Same problem, better insurance."
In one instance of Celtic mythology, the goddess, the Morrígan, heartily recommended that the hero Cú Chulainn surrender to her advances for his own health. He doesn't, and in revenge, she orchestrated his death years later, in the form of a Morton's Fork that forces him to break one of his two geasa, by offering him a meal of dog's meat. (Cú Chulainn was forbidden from eating dogmeat, and never refuse hospitality) By breaking one geas for another, Cú Chulainn lost his supernatural strength and died in battle the next day.
This was attempted in the Epic of Gilgamesh. Gilgamesh turned down Ishtar's marriage proposal for the excellent reason that all her lovers die quickly and horribly after loving her. In response, Ishtar throws a temper tantrum to convince her father, the sky-god Anu to sic the Bull of Heaven, embodiment of the gods' vengeance, on Gilgamesh's city, Uruk, to destroy everyone in it. Predictably, Gilgamesh and Enkiddu slew the Bull, and offered the beast's heart to the sun god Shamash. When Ishtar came to the walls of Uruk to curse Gilgamesh again, Enkiddu came to his friend's defense, and tore off the Bull's right thigh, throwing it at her face while threatening to do the same to her. As a result, Ishtar convinced the other gods to curse Enkiddu with a fatal illness, and thus, grant her some measure of revenge against Gilgamesh for spurning her.
Tsukihime plays this literally; making sexy time is the only way to hold back Roa in Ciel's route. She likes it in the ass, though, so it may not exactly be mating.
And then there are Hisui and Kohaku's routes, as the two girls are "Synchronizers" and can boost the energy and healing of anyone they have sex with. Hisui's first time with Shiki, Lord of Bedroom Jackassery, is used to save him from energy drain and Kohaku's poisons. Kohaku does it to recharge Shiki in her route, as well, and Makihisa's rationale for raping her when she was a little girl was that if he didn't use her power, he'd go insane.
Completely averted in another route: Akiha's, as she and Shiki have sex when she's barely got a grip left on her humanity. Shortly afterward, she becomes the Crimson Red Vermilion, which to Akiha is, for all intents and purposes, equal to death for her, as once a Tohno has lost to their inversion impulse they're no longer human.
In the visual novel of Fate/stay night people can share their magical energy by having sex. This paves the way for Shirou and Saber having sex before confronting Berserker and Gilgamesh on two separate occasions, and Shirou having sex with Rin before fighting Archer in the UBW section of the novel.
This also holds for HF, due to the fact that the worms in Sakura's body and the strain of maintaining Rider are burning up her prana like crazy, which makes her clamor for anything that can restore it. Largely meaning sex. It starts out embarrassing for her and Shirou, but they're in love, so they don't mind. Rather squickily, this is also the reason why Zouken had Shinjiregularly rape Sakura and was implied to have done so himself—training her with Crest Worms massacred her prana supply.
In Dragon Age: Origins, it's revealed that the only way to kill the Archdemon permanently is for Grey Warden to deliver the killing blow, preventing the Archdemon's from body-surfing to the nearest Darkspawn and instead forcing it into the Grey Warden, killing them both. However, Morrigan offers a way out by performing a Blood Magic ritual with either a Male Warden or Alistair/Loghain, impregnating her with a child who will absorb the soul of the Archdemon in their place, but free from all the corruption of the Taint.
Dominic Deegan: Oracle for Hire: In a rather ... controversial flashback, Stonewater rescues Melna, while they're both children, from execution by claiming her as his wife. Unbeknownst to Stonewater, Melna's tribe holds to the tradition that claiming a wife requires immediate physical consummation. As Melna just witnessed both her parents die, she is emotionally hysterical and unable to give effective consent, so Stonewater is forced to rape her. This is shown to be a deeply traumatic experience for both of them.
In Homestuck, the Alternian trolls have a bizarre and brutal reproductive system involving genetic material collection by IMPERIAL DRONES. The brutal part comes in when the drone finds someone who refuses to, or simply cannot, fill the FILIAL PAILS; it simply kills him/her.
Inverted in Grant: it's pretty much Mate Or Kill (in actual fact, simple oral contact with skin will satisfy his Horror Hunger, but it never ends at just smooching).
In Doctor Voluptua, one patient is in this situation thanks to his sentient singing penis, which will eventually take over if not... satisfied.
This trope is the premise of the Livejournal based roleplaying gameAmat Omnes (generally referred to as "Amat"): a Goddess kidnaps characters from their homeworlds, plunks them on an island from which there is no possible escape, and puts on them collars that will gradually choke them to death unless they have sex at least once every two weeks. Luckily for all involved the definition of "sex" is loosely defined to "as long as two people, inappropriate touching, and mutual orgasm is involved".
In Duckman, an ancestor of Cornfed ate a poisoned berry that messed with his DNA, causing all his descendants to die if they wait too long before losing their virginity. Or something.
In one episode of Futurama, Leela and Zapp are forced to have sex by a censorship death star/V-ger homage. It thought they were Adam and Eve, and decided that them mating was the only way to redeem Earth's for its sexually promiscuous sins. If they didn't mate, it would have destroyed Earth.
Inverted with the Decapodians (Zoidberg's species), who are instinctively drawn back to their home planet to mate, and then die.