"Once a girl has been created, circumstances will conspire to keep her a girl."
Not only are male-to-female Gender Benders a lot more common than their female-to-male counterparts, they are also a lot more permanent. One might think that the same Applied Phlebotinum that can change a male into a female should just as easily be able to do the opposite, but that's rarely the case in practice — most characters who try to reverse a male-to-female Gender Bender will learn that Failure Is the Only Option.
There are many reasons for this; even if the phlebotinum is ordinarily reversible, there will be an unexpected Phlebotinum Breakdown, or something else will prevent the character from simply switching the gender-bender into reverse. Perhaps the character inadvertently falls in love with his/her best friend after Jumping the Gender Barrier (the mind is a plaything of the hormones, after all). If there's Time Travel involved, maybe a Kid from the Future turned up to save their life, and the only way to keep the kid from going Ret Gone is to remain a lady (the possibility that they could just have been the kid's father all along is never even considered). Maybe the Gender Bender bestows them with actual superpowers (and no, not thosepowers) that the character needs in order to protect the earth.
But regardless of its in-universe rationale, the First Law is usually enforced because Status Quo Is God: When the original sex change is a central tenet of the premise, then it will never be (permanently) reversed because the series just wouldn't have happened without it; if the series has a finite length, such an event might be saved for its Grand Finale ... or not. On the other hand, if the sex change is not a central element of the series, it's probably just the plot of one-off episode and will be reversed quickly (perhaps to deliver a An Aesop about walking a mile in another "man's" shoes).
Or to put the above another way, if the work is a webcomic called The Amazing Girl-Boy's Adventures in Femininity, expect the First Law to be enforced. If one episode of a Kid Com called Homeroom Genie involves the Jackass Genie turning Zach into Zoe to teach him a lesson? Not so much.
One potential justification for the First Law lies in human genetics (or human genetics run through a Hollywood Science filter): As discussed in Opposite-Sex Clone, you could plausibly turn a man into a woman by replacing his Y chromosome with a second copy of his X chromosome. Turning that woman back into a man would be trickier. You probably forgot to make backup copies of that Y chromosome, didn't you? Without something like a backup, you'd have to create that Y chromosome from scratch.
Other reasons for enforcing First Law stem from Unfortunate Implications and the Double Standard regarding men and women: Women Are Wiser makes for a good gender-equality Aesop. There's also the stereotype that a man being turned into a woman is somehow a demotion, punishment, or loss of status — what better way to start (or finish) a man's Humiliation Conga than by requiring him to wear skirts and a bra while losing that ancient symbol of power under his pants? This is an attitude that even the most feminist Take That against men may support, consciously or unconsciously.
In contrast, female-to-male gender benders are not only less common, but are typically resolved quickly and easily, ultimately reinforcing the woman's original femininity. And since Beauty Is Never Tarnished, the very same forces aiding the restoration of her gender will work against him restoring his.
And of course, since Most Writers Are Male and All Men Are Perverts there's just that much more pulchritude to go around if she gets restored and he doesn't. Sometimes, the gender-bent man's own perversion might contribute as an obstacle against returning to masculinity because he happened to like his existence in the female body if you know what I mean; see Man, I Feel Like a Woman for that. And if the gender-bent man refuses to act in gender-appropriate ways, then who cares? Everyone loves a good Tomboy! (Nobody likes a good Sissy.)
Before you ask, there is indeed a Second Law of Gender Bending, and even a Third Law of Gender Bending. They may or may not overlap with the First Law. This trope was originally named "Mandy's Law of Anime Gender Bending", after the person who first proposed it.
Once Ranma acquires his gender-changing curse, no attempt at curing it works, at least not permanently. Further, many fans have observed that he seems to attract water (which triggers the change) as easily as fiancées and rivals. However, attempts by other characters at making Ranma permanently female also fail, so this may be less the Law than Status Quo Is God, a common trope in author Rumiko Takahashi's works.
The "rules" of Ranma's curse are biased in favor of the Law. "Cold" water will turn Ranma into a girl, but pretty much any temperature below body temperature will suffice. Turning back into a boy requires genuinely hot water. Simply warm water won't do it, so she is usually limited to two options: taking a hot bath or obtaining a kettle of just-boiled water, which Akane seems to supply quite often. Also, the curse seems to make water obey the Rule of Funny and the Rule of Drama. Ranma will generally attract whichever temperature water is the most troublesome at the moment (and since Ranma was born male, this usually translates to cold water and this Law).
Megumi in Tenshi Na Konamaiki has spent six years as a girl at the start of the series, dressing and grooming herself in a very feminine manner. The end of the anime implies (and end of the manga confirmed) that Megumi always was female and her "curse" was a purely mental Gender Bender that had convinced her she'd once been a boy.
Mizuho in Otome wa Boku ni Koishiteru not only gets to stay at the girls' school after being discovered, but volunteers to do so. Except for an obscured view of his face early on, we never get to see him in a 'male' persona.
The manga shows his face in his 'male' persona, and he still IS cute.
Inversion: Sailor Starlights from Sailor Moon. They spend significantly more screen time in male identities, to various degrees, with the handwaving that this makes their act of posing as pop idol singers looking for a lost love more convincing. Of course, the show is aimed at girls...
Kyoji: "Thus Guilt-Na obtained the treasure of "friendship", much more precious than becoming a man."
Guilt-Na:"You're not going to change me back, are you?!"
Ultimately, the way the series deals with the First Law is aversion, at the end of the Manga.
Gacha Gacha Secret has a more understandable case, as the guy is turned into a girl whenever he sneezes, but never even tried to cure it; he just used his girl-form to make friends with the girl he likes to get to know her better. In the end however, it's played straight, as the people that were hired to change him back screwed up because he tried to leave half-way through the treatment so the girl wouldn't lose her friend. This ends up not curing him of the changing, but did cure the condition that would have killed him if he changed too much.
Played with in Happiness!, where Jun is turned into a girl. He's virtually identical looking anyway, and it only causes lots of magic-related difficulties before he decides to turn back. This might be a simple Shout Out to the original game, where it was related to removing a Road Cone from more openminded fans.
Seen progressively in You're Under Arrest!. In the original season, Aoi is originally discovered to wear falsies and the animators make a point to be careful about what outfits they put her in. In later ones, Aoi wears virtually anything the standard girls can wear; we only get the fact dropped in the season premieres, and the requisite Once A Season episode starring him.
Played straight in Pretty Face. After regaining his original face Randou learns from the real Yuna that he'll need to continue posing as Rina's twin for another year, only to be told by Doctor Manabe that if he had another face-changing surgery he'd be stuck with a girl's face for the rest of his life. I Want My Beloved to Be Happy says he'll go along with it. Then the doc pulls off the mask Randou didn't even realize he was wearing, revealing that he actually still had Rina's face. It was a test set by Yuna to see if he was really worthy of Rina. The doctor couldn't bring himself to return Randou to his original appearance in the first place, and instead glued a mask of his original appearance to his face, and continually re-glued it while Randou was sleeping. So, in the long run he ends up spending more time with the eponymous pretty face.
Though he now has the ability to glue the mask back onto his face if he wants to spend time as a guy again.
Manga example: Mashiro Ichijo of After School Nightmare was born male above the waist and female below. Mashiro starts out identifying as a male but struggles with his femininity. Towards the end of the series he accepts his femininity and chooses to be female. The series ends with the revealation that Mashiro's male and female "halves" were actually a pair of Half-Identical Twins struggling for survival in the womb; the female twin lives.
Averted in Shugo Chara!, mostly because Nagihiko is hiding from Amu that he and Nadeshiko are actually the same person.
In Birdy the Mighty, Tsutomu is accidentally killed and has to share Birdy's body, although it can change appearance between the two of them. The Decode remake takes it a step farther, when at the end of the first series he gets his own body back, it is destroyed almost immediately.
Kampfer's Natsuru, after being turned into a girl to take part in a secret tournament of female-only fighters. He is the only character actually afflicted with a gender change, while the others merely transform into violent alter-egos. The change is triggered in the presence of other fighters, and also at will (once mastered), but Natsuru is very very often forced to maintain his female persona.
Played straight on Vandread with Nirvana sub-commander BC, aka Tenmei Uragasumi, Commander of the Taraak Intelligence.
In Knights Of The Dinner Table, the gaming group is forced by their female fellow player-turned-DM to play as characters opposite their normal gender, causing a great gnashing of teeth by the conservatively oriented, hack-and-slash males of the group. When they finally begin playing, most of the players begin "rolling johns" and otherwise using their new gender as a tool for their usual larceny. However, Bob starts to have a deep affection for his character, to the point of preening his character figure. When the group comes across an artifact that would allow gender reversal, everyone leaps on the chance to have their characters switched to males, except for Bob, who states that he would like to have his character remain female. Within a few panels Sarah kills the character off and later admits to the rest of the gang she did it only because he was starting to creep even her out.
Mantra, the title character of the Malibu Ultraverse comic, was warrior knight Lukasz, reincarnated in the body of a woman for the first time after centuries of reincarnating into male bodies.
Marvel Comics' shapeshifter character Courier took on a female form during a time travel adventure with Gambit, as part of a ruse to investigate Mister Sinister (then acting as an obstetrician.) Sinister learned that his patient was a shapeshifter, and managed to extract that ability for his own use. Sinister (not knowing Courier's true sex) inadvertently left Courier permanently female as a side effect of this process.
Mister Sinister himself became subject to the First Law later. After trying to bodyswap with Professor Xavier unsuccessfully, the only available body was a female clone of himself. "Miss" Sinister has since had a fling with Wolverine's son Daken, and is disturbingly attractive considering her origin. However, this character doesn't share Mister Sinister's memories, so it's debatable whether she is "really" him.
Alpha Flight demonstrates that while The First Law is powerful, it can't withstand the power of Status Quo Is God. Near the end of John Byrne's run, the body of Walter Langkowski (aka Sasquatch) was destroyed. Under new writer Bill Mantlo, his spirit eventually ended up merging with the body of the shapeshifter Snowbird, who died while in the form of a white-furred Sasquatch. Walter soon discovered that Sasquatch's new "human" form was, like Snowbird, female, and she lived for several years as Wanda Langkowski. Wanda's sex change was only one of several major changes to the Byrne-era status quo, most of which were reverted when he was off the book, and Sasquatch regained both his manhood and his original orange fur.
Shade, the Changing Man had to take over the body of a drowning woman in order to stay alive himself. Overnight, it reverted to female form and could not be changed back until he found the person responsible for the woman's death. Even after solving that mystery, his female features would re-emerge from time to time until he abandoned the body to fight death on it's own terrain.
Goodbye Charlie: a male chauvinist lady killer is killed by his latest conquest's husband, and returns as Debbie Reynolds.
Switch: a male chauvinist lady killer is killed by his ex-lovers and returns as Ellen Barkin.
Jack Chalker (of course) lampshaded this trope even before it was a trope in Down Timing The Night Side. Once the timestream finds a niche for a nightsider, they become that sort of person in whatever time they leap to. (In the hero's case, a prostitute.) Also shows up in the River Of Dancing Gods series.
Jack Chalker also wrote The Identity Matrix, in which various characters end up wearing each other's bodies. The narrator, a man, finds himself first occupying the body of an Amerindian girl, then occupying the body of a Canadian college student. Since the authorities can't return him to his original body they decide to rewrite his memory to turn him into a stripper and a prostitute. It Makes Sense in Context.
David Weber takes this on in his Safehold series, where the primary protagonist is a robotic duplicate of a young female space-navy officer. When she has to deal with the primitive, male-dominated society that dominates the remains of the human race when she wakes up, she uses the robotic body's nanotech capabilities to turn herself male. And has so far stayed that way through 5 full books. (An amusing moment lampshades the usual Gender Bender trope when she finds herself with a disturbingly physical reaction to what the originally straight, female character sees as several attractive young men... while skinny-dipping...)
In the Wheel of Time series, the Dark One intentionally gives the formerly male Balthamel a beautiful female body as Aran'gar. Getting a body for reincarnation is difficult in the Randland universe, so Aran'gar's choices were either to remain stuck with a woman's body, or die. Again. Being a Depraved Bisexual, Balthamal / Aran'gar was more startled than upset by her change.
In Robert A. Heinlein's I Will Fear No Evil, the rich old cotter protagonist pays to have his brain implanted into a fresh young body in order to cheat death. The first matching donor to come up is a woman. When he comes to after the surgery, he thinks about this a bit and decides it's worth it to see how the other half lives. Seeing as how his surviving the first transplant was a miracle, his situation is pretty permanent.
La Lola is a telenovela that originated in Argentina, and has been adapted in many other markets. Most of them deal with this trope:
In the Argentinian and Spanish versions, Lalo tracks down the witch who turned him into a woman, and she agrees to turn him back (after laughing for a while). They hold hands, she starts her mystic chanting... and promptly falls backward, dead.
The Chilean version gets way more convoluted, as both Lalo/Lola and Pepa/Pepe can't get back to normal and when that happens it's just for a few episodes. In later episodes, there are two Lalos or Pepas thanks to the later-added conditions to the spell.
In the Wingin It episode "I, Carlie", Angel-in-Training (AIT) Denise agrees that Carl, now Carlie, has learned her lesson, and agrees to turn her back into Carl, but finds she can't. The AIT handbook explains that he cannot go back to being a boy until he truly understands girls.
Goodbye Charlie, a sitcom adaptation of the movie of the same name (which was itself based on a play) starring Suzanne Somers as 'Charlie'.
Subverted in Degrassi when Adam, an FtM transgender already living as a male when introduced, detransitions once before reverting to his male persona within the episode and burning his last remaining girl clothes.
Reversed in an episode of Two and a Half Men. Charlie goes to the bar to meet up with an old girlfriend whom he hasn't seen in years. The individual is played by Chris O'Donnell.
Averted by Tiresias, the blind seer of Greek myth, who was turned into a female, and then several years later was turned back into a man.
The movie and television versions of Goodbye Charlie were both based on the 1959-1960 play of the same name. With Lauren Bacall as Charlie.
Both played straight and inverted in the Dating SimKuru Kuru Coeur, in which the three originally female love interests all become male at some point, two staying that way half the game, and all four datable characters have one ending for each gender with the main character changing to match. On the other hand, the originally male main character and his (dateable!) best friend spend 90% of the game as girls and have disturbingly large male fan clubs to boot.
Breath of Fire II has a very interesting case, where the "circumstances" referred to by the Law means the player. Since Spar is a significantly more useful party member as a girl (but loses the transformation if knocked out or zombified in battle), the average player will try to keep the character from losing the transformation if at all possible.
To clarify for those who aren't entirely familiar with the game: most of the playable characters can Unite with Shamans (essentially merging with them) to receive stat bonuses and, with specific combinations, a significant change in appearances and powers. Also note that Spar is a plant and has an effeminate, asexual appearance normally.
Considering that Spar's two other combination forms are an onion with feet (I kid you not) and a plant-like dragon he may be a she to begin with.
In Fable II, there is only one gender reassignment potion in the entire world, and it's permanent and irreversible.
Smug Snake Edwin Odesserion ends up as a woman after using a scroll he thought would turn him into a lich. While he does manage to turn back in a guy later in the game, in the epilogue an encounter with Elminster turned him back into "Edwina", who's now a barmaid (later ending up in Ferelden somehow) and a "bitter, bitter woman".note Since becoming a woman is a possible effect of a scroll mishap in D&D 3.0, s/he may have found an actual lich-turning scroll and just messed up during the casting.
A permanent sex change is also one of the possible results of a Wild Mage's spell failure. While it's possible to reverse the process with another spell failure, given that the game rolls percentage to determine the effects of wild magic, the odds are fairly slim.
Averted in MUDs, where one of the standard status effects is a Gender Bender (which really doesn't do anything in-game except change your pronoun), which affects both genders equally and wears off in a few minutes.
In Misfile, not only does Ash seem to have a better life as a girl, nearly everyone he and the angels have come in contact with since the misfile seem to have had their lives improved in some way. Which hasn't stopped him from wanting his male body back, but as things stand, it's going to be a looooong time before that happens.
It should be noted that this is possibly a case of Unreliable Narrator. Ash, being a teenager, is rather cripplingly insecure, and a careful examination of the plot reveals the female-timeline Ash to have been just as much of a misanthropic teen loner as the male timeline in most concrete respects, with improvements resulting from the presence of his new posse following the change more than the change itself.
Sailor Sun. Despite Transformation Rays being mundane (albeit prohibitively expensive), the Kid from the Future has a tendency to call the main character "Mom," suggesting that she never changes back. The author has theorized that these tropes appeal to people due to the humbling effect inherent in a male-to-female transformation in our male dominated society.
The Wotch: how much Anne loves to turn her male friends into girls is Lampshaded frequently but the tendency seems to extend to everyone with the appropriate powers. Over the course of the strip many, many characters get their gender bent but it's only the formerly male ones who fail to get restored. (Even the straw feminists of D.O.L.L.Y., who arguably earned kharmic punishment, get turned back into girls in the end.) The strip seems to delight in finding reasons why former men can't or shouldn't be turned back, even though given Anne's powers and general benevolence it ought to be easy. There's even a spin-off (The Wotch: Cheer!) comic about four of its permanently genderbent characters.
Also, in the years before the DOLLY arc, though, as stated, male-to-female changes were constant and many were permanent, female-to-male changes never happened except for an oft-bodyswapped couple. One time we thought Anne and Miranda had become boys, but it turns out Wotches cannot be male and so Anne and Miranda are immune to genderbending.
Lord Sykos shows up and goes on a gender-changing spree across town, happily encouraged by his first victim "Aimee". At the end of the story arc he turns all his victims back to normal except Aimee who decides she prefers being Sykos' Perky Female Minion to her former existence. As we don't get to see much of "his" life before the transformation we're given no real motivation for this decision.
Aimee: "Seriously, Anne, don't ruin this for me."
There was also one unnamed guy girl who Lord Sykos sent to get tacos who had the misfortune of failing to return before Lord Sykos restored everyone else and returned to his home dimension, leaving him her with no memory of the incident...and a lot of unanswered questions.
Where am I? Why do I have tacos? ...And why am I a girl?
El Goonish Shive, while not as casual or frequent about it as The Wotch, certainly pays the Law its dues. According to the rules governing the comic's main sex-changing phlebotinum, only male-to-female sex changes can be made permanent (via pregnancy); female-to-male sex changes, even of someone trapped by pregnancy (for whom it's only even possible afterwards), cannot exceed a 30 day time limit. The second time a boy - the main character - is turned into a girl the device breaks, leaving "her" stuck for the full thirty days, and the attempt to get around this leaves him with an Opposite-Sex Cloneand the permanent ability to change sex at will, something the rules of magic eventually force him to do on a regular basis.
Also, much later, a "seyunolu" (chimera) member of the Quirky Miniboss Squad, Vlad, is hit with a Transformation Ray and is turned female, but more importantly to him/her, human. "She" has no desire to change back, and since Bizarre Alien Biology overrides the time limit (and, implicitly, gender identity), Vladia, as she is now called, is treated as a woman from then on.
Moving on from Elliot being a double victim of this, we get to Tedd. Who probably wouldn't know this was a law: the 'circumstances' usually equate to 'Dad's out of town' and 'As a girl, I'm hot'.
Abstract Gender - The guy who has no problems gender changing can change back and forth at will, the guy who hates it is stuck permanently.
Triquetra Cats started the comic this way, but soon delegated it into a minor plot point in favour of a more complex storyline - a new reader not starting at the beginning might take some time to realize they were ever boys.
In Discordia, it's established that anyone transformed by a virgin winds up the virgin's gender; this is true whether they started out that way or not (for instance, a man neutralized via Fountain of Youth ends up as a little girl). Since the only character with such powers is a prepubescent girl, the male cast members had better watch their step. Furthermore, the formerly male main character can't be transformed again at all.
The Order of the Stick introduced The Belt of Gender Bendingnote based on the actual Dungeons & Dragons item, the Girdle of Masculinity/Femininity, which once put on, can't normally be taken off. It requires a Remove Curse spell to get rid of it. And of course, Roy is the only main character to ever wear it.
Averted in the Gender Swap storyline, Helen spends a full week of strips male before Dave is turned into a woman, and they switch back simultaneously.
But once that can of worms was opened, they never could close it again. Dave may not remain a woman permanently, but they transform on many occasions, particularly after Helen and Dave fall in love and start having sex a lot. Now think about that for a moment. It is treated less as a plot point and more as a "thing mad scientists do", along with drugging the coffee and breeding super-gerbils. Also, Dave gives birth to their child. He's even aware beforehand that it's how things will likely come to pass, given that Helen's a mad scientist.
Jayden And Crusader referenced, and subverted, this trope twice, and eventually did swap the gender of its most masculine character. However, that was reversed moments afterwards thanks to his excedingly violent nature. As the entire event was designed for the purpose of gaining readers the trope has only ever been jokingly entertained.
The initial arc of The Dragon Doctors tells how all four doctors - three male and one female - are transformed in an ultra-permanent way. When they've all gotten used to it, the one woman is suddenly turned back in an accident that can't be duplicated for want of a rare and powerful bit of Phlebotinum, leaving all four female; Goro, the one most bothered at first, stated an unwillingness to turn back even if it became an option. Moreover, the doctor having been transformed into a man was already the exception, having been the one to break the curse, hundreds or thousands of men and boys having been transformed in the same ultra-permanent way, several of whom remain supporting characters, and no fewer than two further named male characters (Lee, Frankie) were made female and remain female or died female.
In DDG Netta seems to enjoy keeping Zip a girl far too much to let him ever change back.
In MSF High, Nurse Keiri seems to have made it her mission to warp the gender ratio as much as possible. Everyone who gets more than bandages from her leaves female. At the end of the day, anyone who's been transformed in any way can change back totally or keep all aspects of the new form, forever; with this in mind, she also tends to give incentives not to change back. There's nothing to stop people who have been changed from being changed again, but (other than the daily Reset Button) female-to-male is rare, and never seen in the comic seen once so far and will probably be reversed, where so far at least three significant characters (Urk, Collete, and Victoria) have gone through this progression and are now female for good.
Also, the Legion are a fairly benign version of The Virus; the victims, who seem to have been mostly male, keep most of their personality, but become unquestioningly loyal literal Green Skinned Space Babes. In the main setting, this is as temporary as anything else (although like Keiri, they put some effort into retention), but in most of the galaxy, it's irreversible. (Note that Legion T Fs are only irreversible outside of MSF High because of the lack of transformation magic in Mahou Galaxy. Tends to be all over the place in the Forum RP, as well.)
In Exiern, the main character is a barbarian warrior who is turned into a woman by a misfired magic spell. All his attempts to get turned back to his original gender fail or backfire. At one point, the clerics who were attempting to undo the spell end up turned into women themselves.
Surprisingly, the latter part turns into an aversion of the trope. The clerics perform a reversal spell, and it works... for all of them except the one who obviously loved being a girl, and she obviously refused to reverse her own gender. Though one of the clerics that became male again, turned female again after sneezing, probably implying that their return to manhood is short-lived.
In the photographic novel, Little Worlds, the part of Lev (a male character) is played by a female actor, leading to questions about Lev's gender identity.
When Blair runs amok with the TiresiasOrb in Eerie Cuties, the only major male character affected is outside getting an unconscious boy some fresh air when the orb is destroyed, unleashing a World-Healing Wave on everyone inside the school. He's eventually changed back, but not before spending about two-fifths of the strip's run with his fate in the air, and only by inadvertently transferring it to another important boy, who likewise spends several chapters with his fate uncertain.
In the webcomic Webcomic/Ballerina Mafia, a character's friends give him an Easy Sex Change in his sleep as an April Fool's day prank. He decides to just start living life as a girl now, and was more annoyed than anything by it at first; ultimately, when she's given the chance to change back with no strings attached, she declines.
In The Bend Loro swaps genders somewhat randomly. As a girl, she's a great inventor. As a boy, he's lazy and pretty useless.
If someone is turned into a girl and used to be a guy, no force on Earth can fix it — literally so, as there's a goddess involved. Maybe. It has been hinted that the scientists that came up with this theory — that there's a force of evolution forcing mutants to out-breed humanity — may or may not have been wrong. Ironically, the few characters turned into guys also have trouble fixing it, or don't try. It's just about standard to say that it's easier to cause a transformation then fix it. (See The Big Idea, A Single Fold, and all of the Team Kimba stories.)
Heavily subverted in the character of Jade. She has trouble finding a way to transform herself in the first place! And once she does, failure is the only option... until she resorts to 'traditional' means. Professional surgery that works!
Even appears to be occurring to Phase, who is resisting with all his might the other two. However, it's not certain that it will end up like this, as Phase is still looking through 'outs'.
Subverted in "Ayla and the Mad Scientist": Belphegor gets transformed, and it only lasts long enough for him to dash into the girls' showers to ogle hot babes, whereupon he is restored to his old self... and gets in massive trouble with a bathroom full of superpowered women.
SCP Foundation: (SCP-767 is an artifact capable of transforming males to females only. SCP-113 can go in either direction.) "Experiment Report #113-4: Testing with subjects sex-changed by SCP-767 shows that SCP-113 cannot reverse the sex shift. So far this is the only known case wherein SCP-113 cannot induce a sex change."
Two of the first main characters on The Trading Post were hit with this with different results. Not long after becoming Ashlyn, Jake realized she would not be getting her original body back. Arthur/Liz, on the other hand, thought she was, only to transform into Penelope instead. There have been a couple notable aversions, including Todd and Bryan.
In the Paradise setting, humans are randomly, permanently changed into Funny Animals (with some experiencing a gender-change at the same time). Except for the very few who experience additional changes once per year (and thus have a chance to change genders again), these changes are permanent.
Very common on Literotica when a sex change is involved.
Subverted in the French short film Geraldine (NSFW for cartoon boobies). The main character is a man who turns into a woman. It seems like it's going to stay that way, and as soon as he figures out how to live as a woman, he turns back into a man.
Subverted in Futurama in the episode "Neutopia". At the very end, Scruffy appears to be irreversibly trapped as a woman, but in the next episode he is back to normal.
Earlier though, when the group asked the alien who reversed their genders to fix things, he's ready to go along with it...except Zapp Brannigan chooses that moment to leap in and melt the alien into a puddle, trapping everyone in their swapped genders until the alien's friend comes at the end of the episode to make things right.
Subverted in South Park. During the premier of season 9 Mr. Garrison had a sex change to become a woman, and he stayed that way for three years, at least until season 12 rolled around, when he/she got another sex change to become a man once again.
Subverted in The Fairly OddParents, where Timmy gets turned into a girl for one episode (and Cosmo and Wanda also switch genders), and everyone gets changed back at the end.
Male-to-female sex reassignment surgery is easier to perform, and more closely matches the target sex, than female-to-male sex reassignment surgery. From a tissue-manipulation perspective, it's simply easier to make a convincing vagina out of a penis than the other way around. Inverted by by the hormonal side of gender reassignment. While male-to-female hormone replacement results in a softening of the skin, the development of breasts, and sometimes a change in emotional outlook, most of these can be reversed with female-to-male hormones. (Well, except for the breasts. They usually require surgery to remove.) Female-to-male hormonal replacement, on the other hand, includes many permanent changes such as beard growth, baldness, development of the adam's apple and deepened voice; similar to the permanent changes a male experiences in puberty. Male-to-female hormonal replacement cannot reverse any of the aforementioned effects.