Alice19th: Not sex, but close. Mayura, jealous over Alice and Kyo's relationship, curses Kyo so that if he ever hears a word of love from Alice he will die.
Implied to be Kusanagi's hangup in the Blue SeedOVATime Skip, almost causing him and Momiji to break up because he becomes distant. It's never really resolved in the end either, just sort of alluded to that they will attempt to find a way to work around it.
My Balls: The premise is that a guy had a superpowerful demon sealed in his right testicle, so he has to avoid having sex or ejaculating in any way for a month, or she will be released and destroy the world. This is the cue for every hottie in Hell to come banging on his door.
In Yuria 100 Shiki, Yuria is a Sex Bot programmed to become the unquestioning love-slave of the first person to insert the proper hardware, something she'd prefer not to happen. The alternative is no sex at all. She's a Sex Bot programmed to have sex...
Vaguely used in Black Bird.Later on both Misao and Kyo are given permission because of a curse Kyo receives. For whatever reason, if they don't have sex, Kyo dies.
Fruits Basket: Many members of the Sohma family transform into animals from the Chinese Zodiac if they come into contact with someone of the opposite sex, though the series is never more specific than 'hugging'.
Shigure mentions this in the explanation to Tohru, saying that since hugs cause the transformations, sex is very awkward. It's also worth noting that as Kagura demonstrates, other Zodiac members don't cause a transformation.
Turns out Shigure actually has personal experience in the matter.
At one point, Black Haru comments to Tohru that it's possible to do it without actually embracing, though Yuki and Kyo quickly shut him up before he can go into any specific details.
Over the course of the series, it's gathered that it is very specifically embracing or hugging another person. Kissing is fine (we see that) and so assumedly sex is fine as long as there's no... uh... let's just leave it at that.
Which is to say, the tone of the curse is aimed at undercutting close emotional bonding, not forcing chastity. Though it doesn't help with the sex life, that's for damn sure.
Played straight in Mai-Otome, thanks to the limitations of their Applied Phlebotinum. Akane and Kazuya come this close to averting the trope after eloping, but they keep being interrupted at inconvenient moments, culminating in him getting dragged back to his home country and her getting arrested. Finally, someone arranges a way for them to stay together as master and Otome, but then flat out tells them that this trope still applies.
Okusama Wa Joshikosei plays this one for laughs. The male lead legally marries his student but her father forbids the act with a written contract until she graduates.
It is strongly suggested on the anime's last episode that they finally did it, although this is left open to interpretation by the fact that the girl had often fantasized about this kind of thing before only to be rudely interrupted just like that last time.
Oku-sama wa Mahou Shoujo (don't confuse it with the former) features a character who would lose her powers when she kisses. This may be a case of G-Rated Sex, though the series never says that sex would do it.
Ninja Scroll's Kagero, a female ninja, is so permeated with poison, that her very touch is deadly. A demon by the name of Tessai finds his stone body fatally weakened after attempting to rape her.
Basilisk's Kagero is similar (Basilisk being based on the novel The Kouga Ninja Scrolls, which is among the works Ninja Scroll drew inspiration from).
In Bizenghast, Edrear has an irremovable exoskeleton, which means he can't have sex. Word of God is that it is supposed to deter shipping between Dinah and Edrear (didn't work), but he does shed every three months.
M. Alice LeGrow is adorably naïve if she expected such a ploy to actually work.
While they couldn't mention anything explicit given the age of their target audience, cyborg Guy Shishioh's relationship with his girlfriend Mikoto is visibly strained by the fact that the only flesh-and-blood part left of him is his head.
It's subverted by the last episode.
Explicitly stated in Death Note: Shinigami can't have sexual relations with each other or with humans. Of course, there is a reason for this: If a shinigami falls for a human and then saves him/her from dying, they themselves die.
According to How to Use: XXXVI it's stated that it's both forbidden and actually impossible for shinigami to have sexual relations with humans. This double-emphasis may have been in an attempt to prevent speculation or curtail human/shinigami shipping, though you can guess how well that turned out...
XXXVI also says shinigami cannot have sex with each other, though it is not clear if this is impossible or forbidden.
According to Word of God, Zelgadis from Slayers will injure anyone he goes intimate with, because of his stone body.
In A Certain Magical Index, while Touma Kamijou is very Oblivious to Love, it is pointed out it would be very difficult for him and Hyouka Kazakiri to have a relationship. She's an artificial angel, and as a supernatural being, would be erased if she touches his Anti-Magic right hand, Imagine Breaker. Technically, he could just wear a glove.
Legion of Super-Heroes: Wildfire doesn't actually have a body, having been turned into an Energy Being by a Freak Lab Accident. This puts severe stress on his romance with fellow Legionnaire Dawnstar. Eventually he learns to create a solid body... but it's still made of energy, and Dawnstar gets burned trying to touch him. This is the point where he tells her to just go.
X-Men comics have many mutants have a mutation that makes sex impossible. Examples include Rogue and Wither, who respectively hurt or kill whatever living things they touch. Their partners would effectively need full body condoms.
Wither's case reeks of Idiot Ball as his power only affects organic matter and there are more than enough inorganic characters — one of whom had a crush on him. Instead, he chose to pine for a wholly organic girl who was dating someone else. This didn't end well.
In Rogue's case, the problem wasn't insurmountable: there have always been characters immune to Rogue's absorption power (e. g. the Avengers' Wonder Man in her debut story) and also quite a few who could protect themselves through their own powers (e. g. Magneto in the Age of Apocalypse) or through a machine (e. g. the apparatus Joseph built as a Christmas present). Another way out was exposing Rogue to the powers of mutants (e. g. Leech) or machines (e. g. the one used in the "love grotto" story in UXM #350) or mystical devices (e. g. the Siege Perilous) that can strip mutants of their powers, at least temporarily. In any case, with some help from Professor X and Danger Rogue finally learned to control her absorption power and now can have sex if she wants. Which she eventually did in X-Men Legacy #249 with Magneto.
The Thing has had several lovers since his transformation (The main ones being Alicia Masters and Debbie Green) and this has been addressed a few times. The (overall) interpretation is that he can't have sex because he lacks genitalia, and Stan Lee himself has said that he never really thought about how the Thing could have sex being a giant rock-covered dude and didn't really consider it important in the first place.
This is lampshaded in Rise of the Silver Surfer, when Johnny asks how Ben and Alicia manage a physical relationship, joking that he doesn't want to learn that she died under "a rockslide". All Ben is willing to say is that they have some arrangement worked out.
In The DCU, Heatstroke and Coldsnap are lovers who are members of the villain team the Masters of Disaster. Their motivation for committing crimes is to earn enough money to find a cure for their 'condition' that prevent them from touching.
The Darkness character Jackie Estacado had this problem ever since he turned 21 and acquired his powers (and takes his frustration out on mob goons using Darklings and actually dropping a rabid one down in a Mafia Boss's pants) until they went autopilot and humped an unconscious Sara Pezzini. Usually bearers of the Darkness are male and when a baby boy is born the current owner drops dead. Luckily for Jackie, Sara gave birth to a girl.
There is a loophole, that Darkness bearers can create constructs for this purpose. However this took a turn for the worse when it was revealed that The Darkness intended for this loophole, to encourage the Estacado's to eventually create a construct mate that could bear children. The resulting being would be purely of the Darkness with no human "weaknesses". Once it was born the Darklings mutinied to the creature's side as it tried to become a Self-Made Orphan. It went about as well as you'd expect, but left Jackie without access to his powers for quite some time.
Played for laughs in The Awesome Slapstick, when protagonist Steve Harmon is thrilled that high school beauty Barb Halsey loves his alter-ego, Slapstick. His Black and Nerdy friend Mike then points out that his Slapstick form lacks the requisite equipment...
In the last issue of Crystar Crystal Warrior, with his dying breath, the evil wizard Zardeth punishes Moltar and Lavour for betraying him by restoring Moltar's humanity, but leaving Lavour still a woman made of living magma. And this just after Lavour had finally realized that she genuinely loved him. Fortunately, Lava is Boiling Kool-Aid, so they don't have to avoid each other entirely, but they can never touch.
Seems to be the case with Princess Kavatah in Megalex, at least as of Volume 1. A noble general tries to touch her romantically, and he bursts into flame. She believes it to be a curse.
Jack Hawksmoor of The Authority is fully capable of having sex. However, the process that turned him into the God of Cities also did unspeakable things to his genitalia, to the point that the first woman to see him naked post-conversion threw up on the spot. he eventually hooks up with the Engineer, who has her own set of body issues
In Coming Back Late, Harry and Hermione couldn't do anything physical together because even though she was estranged from Ron, their magical marriage vows were "till death do us part". After Hermione was killed and Harry brought her soul back from behind the Veil, this obviously no longer applied.
In the The Legend of Zelda fic Blood and Spirit, Sheik explains to Link that the Sheikah are forbidden from having romantic relationships because they believe that such emotions cloud their better judgment and rational thinking; thus, they reproduce via birthing mothers, who's sole purpose in the tribe is to bear the children. As a result, the Sheikah children are raised by the entire Sheikah community rather than by their birth parents.
Irena Gallier and Oliver Yates from Cat People. Irena is a werecat, and she and her brother can only sleep with each other without transforming into deadly panthers. However, both of them give in to their urges during the course of the film.
Frank and Lonette from Cool World, because 'noids' and 'doodles' (humans and toons) can't have sex without disastrous consequences.
The 2008 Incredible Hulk movie had Bruce and Betty almost start, at which point his wrist-mounted pulse-reader starts beeping. Bruce stops Betty, explaining that he can't get excited or else he'll Hulk out; she responds, in a disappointed tone of voice, "Not even a little?"
And yet one never heard Bruce Banner say "You wouldn't like me when I'm horny."
In the novelization, it's claimed that Deltans form a permanent mild-meld with their partners, so Decker and Ilia would wind up as a two-person Hive Mind. Most humans find this idea terrifying. Moreover, Deltans constantly produce pheromones that make everyone want to sleep with them even if they know better. So Deltans serving in Starfleet have to take an oath of celibacy.
In more recent novels, such as "Watching the Clock", it was toned down to a temporary mind-meld, but Deltan emotions are even more potent and volatile than Vulcan emotions, and the mental connection during the act caused them to be amplified even more. Non-Deltan's don't always have the mental strength to experience Deltan emotions, without eventually going insane or just ceasing to function.
MAD Magazine's take on it:
Ilia: I am Lt. Ileer! And before any of you get any ideas, let me remind you of my oath of celibacy!
Kirk: With that bald dome of yours, lady, I wouldn't sweat it! The only thing that may attack you on this ship is a bowling ball!
In a later panel, a bowling ball rolls on the scene, asking, "Can Ileer Come out and Play?"
In Bent, the main characters are a gay couple in a concentration camp. They are incapable of touching each other, standing within a foot of each other, or even looking at each other. What they can do, however is have incredibly emotional and vivid verbal sex.
Partial trope in The One Who Waited, Alice and the Boogeyman can technically never have sex, except at the very end, when the act of doing so kills Alice
Debatable: Either the house fire, or the sex kills her. It is left to the interpretation of the reader.
In the Incarnations of Immortality novel "For the Love of Evil," Parry has become a Friar in the Middle ages. He has a very strict celibacy oath to which he must adhere to remain a friar. He had a beautiful, intelligent wife (Jolie) who was murdered, and is now a ghost. He still has a very high sex drive. One night, he has the opportunity to have sex with Jolie, thanks to the intercession of a willing female host. Suffice it to say, his intense performance that evening sets the plot of the entire rest of the novel in motion.
In Piers Anthony's Apprentice Adept series. Stile's wife has been prophesied to have one child by her second husband. So as long as they don't consummate their marriage, Stile remains safe from death. They eventually do consummate after the third book, and they have their son, but since the prophecy does not specifically mention imminent death after that son, Stile and his alternate reality counterpart, now trapped in opposite worlds, continue to be major characters for the rest of the series.
Cal Leandros is so afraid of impregnating the human he loves (he doesn't trust birth control at all) with his half-evil-fairy sperm that he will only boink other species that he's biologically incompatible with.
Thomas in The Dresden Files books can no longer even touch his beloved Justine - or even a scarf she made by hand - without being burned, because they truly love each other, and true love is holy water to White Court vampires like him. He's free to do anything he wants with anyone he doesn't care about and in fact drains the life force his demonic side needs through acts of lust.
It's even more difficult for Justine, whose protection against the White Court lasts only if her last sexual encounter was with her true love, i.e. Thomas. She loses protection if she gets with anyone else.
They do find a solution that would make The Harlem Globetrotters proud. In a nutshell: Justine must first have sex with a third party prior to every encounter with Thomas (undoing the true love issue). This allows her to remain protected from the White Court because the only times she won't be protected, Thomas (freshly fed/recharged from sex) is right next to her, thus endlessly renewing the protection.
Susan is also an example: after she becomes a vampire, she can't have sex with Harry because she can't separate her desire for him from her desire for ripping out his throat and drinking his blood. Understandably, this puts a slight damper on their relationship. However, they end up getting around it by tying her up with enchanted rope.
This is a frequent problem in Anne McCaffrey's The Ship Who... series, where the protagonists are very handicapped humans who operate as the "brain" of a ship. They can't get physical when they are an immobilized body in a column. Averted halfway through the series, when one brain deals with the problem by commissioning a remote-controlled full-sensory human body, opening up the same possibility for other brains.
Also mentioned in the books is that one of the reasons the administrators will reassign brain-brawn pairs if they click too well is that there have been a few cases where a brawn has decided enough is enough and cracks the column to get at the chewy center inside (for this very reason, brawns are absolutely not supposed to have the access codes to the column, but sometimes obsession finds a way).
Confessors from the Sword of Truthcan have sex, just not with anyone they actually like, because they accidentally release their power while love making, and it would essentially destroy their lover. This causes much drama for Richard and Kahlan in the first book. Once this is resolved, they have another problem because of some prophecy that she'll give birth to a male Confessor (who are Always Chaotic Evil).
Subverted in Spider Robinson's Callahans Crosstime Saloon series. Michael Finn, a humanoid alien cyborg, believes he can never have sex because his superhuman abilities would cause him to accidentally injure or kill whoever he has sex with. One of the other characters points out that he can still administer oral sex, and be given hand jobs.
Discworld wizards can't have sex because there is a risk that the resulting child could be a sorceror, a magician powerful enough to destroy the world by accident. They don't seem to have heard of non-procreative sex.
It's implied, at least in the earlier books, that sex uses some of the same circuits as doing magic does, for wizards at any rate. In "Mort", it's noted that once Igneous Cutwell starts spending time with Queen Keli of Sto Lat, he doesn't do any magic any more: this is clearly a big hint that they are getting it on instead.
This is very much not the case with witches though: Nanny Ogg and Lilith Weatherwax
Judging by Moving Pictures, at least, the idea seems to be to never ever tell them anything about sex so that they have no idea how it works. If they don't know how to have sex, they won't, and no sourcerors will show up!
Good luck hiding that from the wizards raised in rural environments, especially Archchancellor Ridcully. Well, Ridcully himself averts this. Despite his rural upbringing, he is actually extremely sensible, especially as he develops. He actually understands the reason for no sex and keeps himself proper by pursuing only the occasional platonic relationship such as with Granny Weatherwax.
It's actually worse than not having discovered non-procreative sex; a sourceror will only be born if they're the eighth son of an eighth son of an eighth son. So in theory, there's lots of sex that can be had before they have to stop, and some wizard could never have a problem. This is mentioned in some of the early books, so there's a good chance it's the kind of canon that can be safely ignored.
This is then possibly averted in Making Money, where it's revealed several necromancy students only want to be necromancers because they get the official skull ring which they claim is a 'babe magnet'. Technically it's only marriage that's forbidden. There's presumably only a problem if all eight children are with the same woman.
In Unseen Academicals it is mentioned that Professor Macarona, at Unseen University on exchange, has apparently left a trail of angry husbands and at least one angry wife in other cities he's visited. Nothiswife, you understand. Apparently celibacy is an Unseen University tradition rather than one that necessarily applies to all wizards, and there are indications that even UU is occasionally willing to look the other way as long as you're not too blatant about it.
In the first book, Rincewind is describes as looking like an "apprentice enchanter who had run away from his master out of defiance, boredom, fear and a lingering taste for heterosexuality." Make of that what you will.
In The Light Fantastic Rincewind knows what orgasms are - he's had a few. Sometimes in company. His first use of real magic is compared to a good orgasm, and most wizards can use magic when they want.
The tendency towards celibacy is pretty deeply ingrained in at least some of the wizards. Rincewind at one point finds himself as the only male on an island full of half-naked Amazon babes who offer to give him anything he wants, no matter how depraved or lascivious. He's literally drooling in anticipation of the satisfaction of his desires. He asks for potatoes.
Played on a villain in Exiles at the Well of Souls, when the sexually-perverse and licentious Antor Trelig is transformed into a Makiem, a frog-like alien race. Makiem don't copulate at all, they just release gametes into the water once a year without any physical contact. Karma's a bitch, eh?
In Isaac Asimov's novel The Gods Themselves, it is emphasized a few times that Earthborn and Moonborn people suffer from a slight sexual incompatibility due to Earth people's tendency to subconsciously move as in normal Earth-gravity during moments of abandon—an "Earthie" would be very likely to injure their partner in the lower gravity of the Moon. In the end, it's implied that the protagonists can work something out.
It's indicated to be a curse on the Mai. In the end of the books, the curse is broken. The TV series never made it that far.
Cal from Peeps has a sexually-transmitted parasite that gives most people who pick it up vampiric traits, something he didn't figure out until his ex-girlfriends went crazy. This is eventually subverted when (a) it turns out that passing on the parasite is a good thing, (b) the craziness can be cured easily for those that it happens to, and (c) the girl Cal likes eventually picks up the parasite from his cat (cat breath is another vector) instead.
The hitek, a near-future offshoot of humanity from Man After Man, are unlikely to survive having sex, as their bodies are so crippled by the accumulation of genetic defects that they need constant life-support to function. A hitek couple in the book require the approval of their physicians to even make the attempt, and when the female dies of heart failure from the exertion, the bereaved male crawls back into his life-support cocoon, never to emerge again.
In Louise Burton's Hidden Grotto novels (House of Dark Delights, Bound by Moonlight), there are two characters in love who may never touch - Elic, a dusios (type of sex-changing incubus), and Lili, a succubus. They find ways around not having sex with each other...
This trope heavily figures in The Sun Also Rises by Ernest Hemingway. Jake Barnes is very much in love with Brett Ashley. She even reciprocates! However certain practical (and metaphorically mental) issues preclude any consummation.
In the last book of The Emigrants tetralogy Karl Oskar and Kristina can no longer have sex since they have no reliable birth control and if Kristina gets pregnant again it will lead to her death.
In The Vampire Chronicles, once you're a vampire, you can't have actual sex. However, everything else practically becomes a substitute. Even the pattern on a carpet can bring rapturous pleasure to one's enhanced senses.
In a Polish vampire trilogy Nocarz, this can happen to a vampire couple if one of them gets neutralised with Sator's anti-symbiote vaccine. The victims not only lose their vampiric traits, but also react with deadly allergy to any physical contact with a normal vampire (it's implied this effect will pass with time). Knowing this, Vesper assigns his human friend to provide medical first aid to the neutralised vampires, as any other member of the team would kill them with barely a touch.
Echis: How could I argue with you, when you could kill me just by spitting in my face?
After Icta gets neutralised, Vesper imagines her having a normal life, but decides that Echis would make a better husband and protector than him. It's implied that it will work out in the future, unlike the rest of his hare-brained plan.
There's a cat character in Clare Bell's Clan Ground series who's like this. He's half Unnamed, and as most of the Unnamed are dumb, brutish, unintelligent cats, he fears passing on his Unnamed genes to any potential offspring. So, he leaves the clan during breeding time each year.
Every time Laurent and Thérèse try to have sex after they get married (or even try to sleep for that matter) in Thérèse Raquin, they are haunted by memories of Thérèse's first husband Camille.
In Warrior Cats, medicine cats are forbidden from having kits, so they aren't allowed to have mates. That's not to say they don't break the rule, but for the most part they adhere to Clan standards.
In Tales of Kolmar, Kantri have incredibly high internal temperatures. A human once helps one of the Kantri deliver a baby, has to reach in and turn it, and the flesh on the human's arms is so burned it comes off in rags. Humans and Kantri are not sexually compatible. The very idea that they could be is seen as absurd; there is a prophecy of human-Kantri children spelling the end of the world as we know it, but someone outright says that there might as well be a prophecy warning them to beware of a bull and a butterfly. So when the Lord of the Kantri falls in love with a human woman, they can't consummate. Until he's turned into a human himself.
In Tim Dorsey's Serge Storms books, Johnny Vegas is always interrupted, sometimes by incredibly unlikely things, just before he's about to have sex.
In Dave Duncan's "A Man of His Word" books the Sultan of Arakkaran, Azak, is cursed so that he can't touch a woman without burning her badly. Even after the sorceress who cursed him dies the curse is not lifted, cutting his wedding night short, marking Inosolan and leading to a lot of frustration.
Undead in Kevin J. Anderson's Dan Shamble Zombie PI novels apparently do retain the ability to have sex. Dan, however, can't have sex with his girlfriend Sheyenne because she's a ghost, unable to touch anything animate. In Unnatural Acts, they find a stopgap solution: Sheyenne can manipulate inanimate objects, so she wears gloves to hold Dan's hand or possesses a blow-up doll for intimate relations.
Buffy the Vampire Slayer: Angel had one night with Buffy, which enabled his Curse Escape Clause and led to his infamous Face-Heel Turn as Angelus. Angel has moved on to boink other women he doesn't love, but he can never close the deal again. Well, until the After the Fall comic books, but no women he loves are around and he's too busy by that point.
Played with on Angel with Cordelia and her beau, the Groosalugg. She has reason to believe that having sex will lead to her losing her precognitive powers; after spending time bemoaning, her response is to find a magical prophylactic so that won't happen.
Angel eventually has the characters work out that Angel's curse isn't the No Sex Allowed curse they originally thought but is, in fact, this trope played straight. In series 1, When Wesley and Cordelia discuss the idea of Angel and Rebecca Lowell having a relationship, Wesley points out the curse's requirement is "perfect happiness" and also points out how rare that criterum actually is. In series 2, Angel can get away with having sex with Darla, who he has complicated emotional ties to, because he experienced "perfect despair" with her. In series 5, Angel doesn't lose his soul by having sex with Eve because he doesn't trust her and they were under a mystical compulsion. Wesley reiterates that "perfect happiness" is very rare and most relationships are formed from "acceptable happiness" instead which is why Angel can have sex with Nina without his soul being endangered. As a result, the only people Angel Can't Have Sex, Ever with are the women he regards as his true loves (and the ones he truly wanted to be with): Buffy and Cordelia.
Becomes a running gag throughout Angel, with Angel himself having to clarify more than once that he's "not a eunuch."
And then there is Gwen, whose electric powers are always on, forbidding her from touching people. Or, at least, alive people. She does get around this later, however.
Every pairing in Lexx suffered from this. Afflicted characters included a dead man who lacked certain parts, a severed robot head with no moving parts, and an alien who was "smooth round the bend". And that was before the plot device of the ship's coveted key — a symbiotic energy life-form — abandoning its host at the height of ecstasy...
And the writers loved to taunt the audience with it, too; when 790 gains a working arm, thus becoming a robot-head-with-arm, he tries to get his new hand in Xev's pants...but she's too tired from previous events to care and 790 loses the arm shortly thereafter. 790 would again come close to getting some actual action when Prince arranged for his head to be attached to the body of a moth breeder who had the key to the Lexx, as well as going through the United States military's service record to find the most well-endowed soldier to volunteer an equipment transplant to the moth breeder's body. Before 790 could get his freak on, his head was knocked off, leaving him bodiless again, and Xev went to "find" the moth breeder.
In an earlier episode, 790 attaches himself to a headless cyborg body found in a prison ship. However. reactivating the body causes 790 to be intermittently possessed by the personality of the body's previous owner, a Depraved HomosexualScary Black Man rapist who immediately sets his sights on Stanley.
Dark Angel loved doing this to Max and Logan. At first they couldn't do it because Logan physically couldn't (he was paralyzed from the waist down). Then Logan gets better, but Max has been infected with a designer virus made especially to target Logan's DNA — so any skin-to-skin contact with her would kill him. Then the show was canceled.
Alisha from Misfits can't have mutually consensual sex with anyone due to her power (she's basically a walking Love Potion, whoever touches her is overcomed with uncontrollable lust).
At first she doesn't see the problem with her ability, which basically means she can have sex with whoever she wants and given that she's a Femme Fatale that works out great for her, until she's almost forced into sex by two men due to accidentally touching them. As from then, she realizes that forcing people into sex without their actual consent is not right and does not have sex anymore. Until Seth takes away her ability in the Series 2 finale, that is.
In season 2 of Torchwood, after Owen dies and is resurrected, becoming a moving, thinking corpse, he finds that he can no longer have sex. This is the time that Toshiko finally confesses her feelings to him...
Nick and Natalie venture into this territory in Forever Knight, because sex and feeding are very much tied together for vampires, and Nick is mostly unable to control himself once he starts feeding. He knows he'd either wind up turning Natalie or more likely, that he'd kill her.
In Legend of the Seeker the main characters, Richard and Kahlan can never consummate their love, as Kahlan's Confessor powers would be unleashed in the throes of passion which then would leave Richard "Confessed" or a slave to Kahlan's will. There are numerous flirts with intimacy, but the trope holds fast. Until the second season finale, that is, when Richard and Kahlan discover that she can't Confess him because he's already so deeply in love with her that it makes no difference.
In Haven, Nathan Wuornos has Feel No Pain, and the only person he can feel is Audrey Parker and her past incarnations like Sarah Vernon. He can have sex with other women, but it's obviously not very enjoyable. Jordan McKee inflicts agonizing pain on anybody who touches her bare skin, so only Nathan can touch her (Audrey can too, but both girls are straight).
Being Human features a strange case with Mitchell and Annie, who have two problems: first, Annie is a ghost, so the only way for them to touch each other, not to mention have sex, is for her to possess Mitchell's partner. The second problem is that Mitchell's a vampire, who, in the show's universe, are walking metaphors for drug addicts, and finds it hard to separate sex from feeding (and he very much doesn't want to drink human blood). Eventually they decide to forgo sex altogether. It's not clear whether it works as Mitchell dies pretty soon after.
In both Werewolf: The Apocalypse and Werewolf: The Forsaken,, werewolves mating with each other tends to have bad results. The Apocalypse gave us Metis, the sterile, deformed offspring of Garou mating - the only "upside" is that they're born in the Wolf Man Crinos form, giving them more power. The Forsaken changes it so that the child of two Urathra is a unihar, a vicious, powerful spirit bent on killing its parents and any werewolves that get in the way. Though this is more "Can't Have Sex With Own Species"; werewolves haven't gone extinct yet because they're interfertile with humans (and, in Apocalypse, wolves).
There's also a popular misconception that vampires in both Vampire: The Masquerade and Vampire: The Requiem can't have sex. This is not entirely true. They are capable, but it is seen as a waste of Vitae (since they need to expend some to perform the deed), they hardly have any sex drive and the Blood Lust tends to over-ride the regular lust every time. But what really kills it for a lot of vampires is that blood has replaced all their bodily fluids. All of them. So vampire sex with a mortal would either be highly disturbing or a breach of The Masquerade and sex between two vampires is guaranteed to ruin all the bedding forever.
In Warhammer 40k, having sex (or any excessiveness of pleasure) as an Eldar will result in your soul becoming the newest sex toy for the Chaos God Slaanesh, lord of sensations and excess.
Darklord Ivana Boritsi of the Ravenloft setting has turned both herself and members of her entourage into ermordenung: living humans whose bodies are permanently saturated with a deadly poison. Ermordenung can't touch anyone normal without killing them, and Ivana cranks up their frustration by using a different poison to create each one, so they'll kill each other if they touch their own kind. Bitch. (To make this worse, the leader and most powerful of the ermordenung, the first who received this "gift", was Nostalia RomaineIvana's best friend as a child, and it was with her help that Boritsi seized control of Borca, using Natasha as an assassin to kill her mother, the previous ruler. Despite this, Natasha and the others are still utterly loyal to her; of course, when you work for a darklord, you tend to be just as evil as your boss.
And any attempts Jacqueline Renier has ends when she inevitably turns into her wererat form and kills them, she can't control this. She also arranges for her sister's lovers to be murdered before they can consummate their relationships out of jealousy (and to keep Louise from usurping her power). Jacqueline's curse only applies if she actually loves her prospective partner. She can have all the casual or Foe Yay sex she wants.
Sir Tristen Hiregaard of Nova Vassa isn't a straight example of the Trope, but he's to Renier similar. He can have sex if he wants (and he has several sons from his Arranged Marriage to show for it) but any woman he actually loves is in mortal danger. A curse he suffers from transforms him into a fiend called Malken (the true darklord of Nova Vassa) a madman who targets any woman Hiregard loves. (Unlike Renier, Hiregaard isn't malicious, and doesn't truly know that he is Malken, but he does know that there's a dark presence inside him, and tries to avoid becoming attached to women because of it; it's often hard to find any who are attracted to him nowadays anyway, as they tend to remember what happened to his previous lovers.)
Subverted in Warhammer Fantasy with the Dark Elves. To save his life the Witch King Malekeith had a magical suit of armor fused to his body that grants him to strength to fight, and makes him hard to kill (both in the background and in game). However, he believes only a male wizard using dark magic can kill him, especially since most of his protection don't protect against magic. So he has every male sorcerer in his empire killed off, forbids males from learning magic and has them hunted down, and has all sorceresses married to him to ensure that they do not have a child (he's fused to the armor, it ain't coming off). It's a subversion because many of the Witch King's Brides take on male lovers, and sometimes have children in secret.
Both love interests in Planescape: Torment suffer from this. Grace drains the life force of anyone she kisses. Annah heats up to dangerous temperatures once her blood starts racing.
And since the main character is immortal, it doesn't matter one bit.
Quarians in the Mass Effect universe are generally like this with other races — thanks to their weak immune systems, sex is dangerous even between two quarians. However, Tali, the quarian love interest in ME2 actively works to subvert the trope if you're romancing her. By the third game, her immune system has adapted to Shepard and the two can have sex without issue.
Turians have a similar issue — being dextro-amino-based lifeforms, any sort of fluid-exchange could (in a worst case scenario) cause a deadly allergic reaction in either party, and Mordin's repeated warnings of chafing. Once again, you can still go with it if you're romancing Garrus, but it takes quite a bit of planning beforehand. Mordin suggests that you try not to ingest.
Some asari are Ardat-Yakshi: they suffer from a condition that fatally burns out the nervous system of anyone with whom they mate. To make matters worse, this also gives the Ardat-Yakshi a boost to their own natural biotic ability, which proves addictive.
Your brittle-boned pilot Jeff "Joker" Moreau is implied to be unable to have sex unless willing to risk a shattered pelvis due to his disease. He even claims "light, over-the-clothes action" carries risks for injuries.
Isabella "Ivy" Valentine, the whip-sword fighter from the Soul Series, swore a vow of celibacy so that she may not pass on her Malfested curse to any of her potential children. This wound up being a valid concern, as both of Sophitia's kids wound up with their fates cursed by Soul Edge and Soul Calibur. Ivy vents out her sexual frustration by being generally dominatrix themed.
Yasu/Shannon/Kanon/Beatrice from Umineko: When They Cry is said to be unable to have sex due to the injuries and mutilation of his/her genitals after being thrown from a cliff as a baby. This together with his/her confusion about his/her gender and sexuality makes him/her consider him/herself as "furniture" and not human. This only gets worse since each of his/her Split Personalities fell in love with a different person, and one of them talks frequently about marriage and children...
Way too many characters in the Whateley Universe. Fubar has been turned into a water-breathing thing like a Star Spawn. Puppet's secretions are an incredibly deadly poison and also she's on a machine to circulate her 'blood' and drain her 'lymph', so she lives alone in a room she never gets to leave. Compiler's nanites give her superstrength and superspeed she can't control. Diz can't even touch people because she has a force field that exerts eight tons of force all the time and she can't turn it off. Antenna generates so much electricity he's like a walking lightning storm. And there are more, just on the campus of Superhero School Whateley Academy.
In Worm Newter's bodily fluids are contact hallucinogens, to the point that even trace sweat on his skin can render someone incoherent. While it's his main weapon in a fight, as one reader points out, this poses certain difficulties in the bedroom.
Tattletale also avoids sex because her Hyper Awareness causes sensory overload.
Sam's species in Freefall dies whenever they have sex.
They also don't gain full sapience until after the species equivalent of andro/menopause. So their society is made up entirely of those who either couldn't or wouldn't breed when they had the chance.
In The Inexplicable Adventures of Bob!, it's never been spelled out, but it's been heavily implied that, however effective Voluptua's shapeshifter unit that lets her appear human may be, she and Bob would not be able to do much of anything physical, even if they were so inclined.
Spring goddesses in MYth face their doom when they have a child, since the goddess passes her powers to her child and then dissapears to return to Gaia. Demeter suffers this after giving birth to Persephone and Hades is aware this could happen to Persephone too.
In Futurama, after members of Zoidberg's species mate, they die. Averted because this doesn't stop them.
Brock Samson (of The Venture Bros. fame) can never go past second base with Molotov Cocktease (the only woman he ever loved) because of her chastity belt.
A more PG version in Adventure Time with Flame Princess—it turns out her Emotional Powers are unstable and romance could cause her to burn through the crust and destroy the Earth. Unfortunately, neither she nor Finn realize that before they decide to have their first kiss.