It's hard to tell what does it exactly. Maybe it's the tendency of good guys to have larger Johnsons than their evil counterparts. Perhaps it's the realization that Good People Have Good Sex. But for some reason or other, sending an evil woman, especially a Dark Chick, to seduce a hero has a surprisingly high turnover rate. There's a very good chance that's she'll just decide to bat for the other team (moral team, that is) rather than stick to the original plan. The sexier the hero, the more likely this will work.
To make some sense out of this trope, look at this way. Sex Equals Love right? And Love Redeems, right? So sex redeems. If you're a woman, anyway—this is rarely used with female heroes, making this something of a sexy Double Standard.
The trope may have a foot on reality's ground, for less than intuitive reasons: usually the villain is a villain for everyone, this including his or her female agents (who may get it even worse in their own camp compared with other minions of the Big Bad, for being at the same time female, spies and whores). If the good guys prove their own goodness in bed, but at the same time in their manners, way of acting and treating her, this may be the last straw to convince her it's the proper time to switch boats.
The trope can also be gender inverted, with a male (usually the protagonist) suddenly being a lot more interested in helping some group after sleeping with a woman who's in the group.
For more wordplay on related tropes, think of this as Sex Equals Redemption as opposed to Redemption Equals Sex. Here, the redemption comes after the sexual encounter instead of before.
May or may not be a part of a High-HeelFace Turn, In Love with the Mark or Romantic FakeReal Turn. Very common as a result of Go Seduce My Archnemesis, markedly less so in a Honey Trap. If the sex itself doesn't cause redemption, but she gets knocked up and that does, it's Deliver Us from Evil. Also see Villainesses Want Heroes which is basically the same except she doesn't succeed. If initiated by a female character, she is always a Heroic Seductress.
- Okiku from Afro Samurai. It helps that she and Afro were Childhood Friends.
- This is subverted/played around with in GUN×SWORD. Michael Garret performs a FaceHeel Turn and got along well with Fasalina. Eventually at one point, Fasalina opts to have sex with Michael, and while they still do not make their HeelFace Turn officially, that event was a turning point that they do love each other and have other important thing between each other aside of serving The Claw. In the ending, it almost looked like they could stop being evil, until their relationship came to a rather rocky end. However, this granted them enough sympathy points that in Super Robot Wars K, they survive that event and finally perform their HeelFace Turn there (provided that certain steps are taken).
- Subverted in JoJo's Bizarre Adventure. Polnareff gets the idea of making Jotaro seduce Midler into sparing them after the woman in question admits having a Villainous Crush. Unfortunately, while a Chick Magnet he may be, Jotaro is Not Good with People and even worse with romance.
- Satan becomes an angel again after being "purified" by Michael in My Balls, complete with Immodest Orgasm and a Pillar of Light. It's that kind of series.
- In Yu Yu Hakusho, after a female human doctor healed the demon Raizen, he intended to devour her. She calmly retorted that her body was saturated with deadly poisons she had ingested to produce primitive vaccines in her own blood, enough that even a demon would be killed. Raizen, mesmerized by her fearlessness made love with her and departed the next morning. Having spent his life killing, Raizen felt unworthy of her and refused to eat humans from that day on, even after her Death by Childbirth, starving to death over several centuries while hoping he would meet her reincarnation.
- The original, 1940s Golden Age Airboy series. Kissing the titular protagonist didn't turn Valkyrie, a Nazi air ace, against her superiors - finding out that her subordinates were about to be executed did. Unfortunately, many fans of the later 1980s incarnation of the series didn't have an opportunity to read the original stories, so they assumed that this was an example of the trope.
- In the Mystique series, Mystique slept with a seeming ally, Shepard Leighton, only to have him betray her to Mystique's foe, the Quiet Man who was actually possessed by the mind of Shepard's dead sister.
- Gender-flipped in Ultimate X-Men, in which Wolverine is sent by the Brotherhood of Mutants to infiltrate the X-Men and assassinate Xavier, but defects for real after entering a sexual relationship with Jean Grey. She is not happy when she finds out.
- Gender inverted in Undocumented Features: Symphony of the Sword, where Brainwashed and Crazy Saiyonji is redeemed by Azalynn having sex with him. Of course, for her, it's a religious ritual, and was also accompanied by talking him out of his craziness.
- This is still happening with the one of the main character of "The Blackened Heart".
- In the Death Note Slash Fic A Cure for Love once Light becomes Kira again he's much more reluctant to kill L now that they've been sleeping together.
- Likewise in Fever Dreams, after having sex with L, Light decides he doesn't want to kill him and so he has to quickly rethink his strategy in order to negate the traps he'd set and cover his (and Misa's) tracks.
- Subverted in the Kung Fu Panda fanfic The Vow: after Lord Shen forgives his former fiancée Lady Lianne of the betrayal she never did thirty years earlier, they rekindle their love and copulate. However, Shen stays true to his canon goals, telling to the shocked Lianne that nothing else than their relationship has changed. However, after he marries and subsequently releases her, he cheats death and is forced to live as his wife's secret prisoner, but still with her and their son.
- Austin Powers: The Spy Who Shagged Me. Ivonna Humpalot and Austin Powers.
- A major reason for Jake going native in Avatar is the fact that he's boinking a very hot alien chick. The Colonel even calls him out on it.
Quaritch: "So, what, you find yourself some local tail... and then you completely forget what team you're on?"
- Lili Von Shtupp from Blazing Saddles pulls this after being surprised and overjoyed at how big the black Sheriff Bart turns out to be. "It's twue, it's twue!" Had the censors and / or executives not objected, this was to have been played with for parody; Bart's next line after this was to have been to politely point out that Lili was, in fact, sucking on his elbow. ...which is very interesting, because as written it subverts and mocks the stereotype, but as played it reinforces it, making it more bigoted—-showing where the executives' priorities were.
- James Bond examples:
- Tatiana Romanova in From Russia with Love. She is assigned to seduce Bond in the first place as part of a Honey Trap but really falls in love with him after she is successful.
- Pussy Galore in Goldfinger, which is a turn in more ways than one; it's implied in the film that Ms. Galore is a lesbian before her encounter with Bond (that's explicitly the case in Ian Fleming's novel). This is one of the less convincing versions, so it's likely this was why the trope was averted in the following movie:
- Lampshaded and mocked spectacularly in Thunderball by nasty girl Fiona:
Fiona: But of course, I forgot your ego, Mr. Bond. James Bond, who only has to make love to a woman, and she starts to hear heavenly choirs singing. She repents, and turns to the side of right and virtue...[she steps on Bond's foot]Fiona: ... but not this one!
- Tiffany Case in Diamonds Are Forever.
- In Live and Let Die Solitaire may have actually been attracted to him before it happened. (She may have secretly sent him a warning that the first girl worked for Mr. Big) and truly switched sides after he seduced her. (He tricked her - possibly. He later told her that the deck of tarot cards he used was rigged, but she was neither upset nor surprised; it was clear she held at least some grudge against Mr. Big already.)
- Subverted in A View to a Kill. Bond does sleep with Zorin's main henchwoman (and lover) May Day, but when she eventually turns against her boss, it's because Zorin knowingly left her to her death, not because she's smitten with Bond's manly ways. In fact, May Day could be said to be manlier than Bond himself.
- Averted in Die Another Day, in which Miranda Frost has sex with Bond to distract him so that she can convince him she is still on his side and remove all the bullets from his gun.
- Played with in North by Northwest. Eve clearly falls in love with protagonist Roger when she is ordered to seduce him by her boyfriend Vandamm, but shows no sign of actually leaving Vandamm for him. Then it turns out she's actually The Mole for the federal government and is staying with Vandamm so she can keep providing information for them, subverting the trope entirely.
- Gila in Our Man Flint, with Derek Flint.
- In The Pink Panther Strikes Again, the same effect is achieved not by the antagonistic woman actually seducing Clouseau, but by having her sleep with someone she thought was him when it was in fact another member of the same multi-national Carnival of Killers as her. This leaves Clouseau very confused when she suddenly appears naked in his bed professing her undying love for him.
- Revenge of the Nerds: Betty is more of the Alpha Bitch than an actual enemy, but she IS the Alpha Beta leader's girlfriend until Lewis Skolnick covertly has sex with (i.e. rapes) her in the Moon Room during the Carnival.
Lewis: Jocks only think about sports. We [nerds] only think about sex.
- Averted with Loretta in The Sting. She sleeps with Johnny Hooker because she's the hitman "Salino" who's been hired to kill him. Somewhat played with because the character the audience thinks is the hitman is actually a bodyguard hired by Gondorff for Johnny's protection. It doesn't hurt that up until the very last second, the audience thinks that the only person who would help Johnny has just been shot by the 'man' gunning for him.
- Achilles in Troy with Briseis. He gains a heart and purpose from there on.
- Undercover Brother. White She Devil and the title character.
- Sorsha from Willow with Madmartigan.
- In Dragon Bones, nobleman Garranon sleeps with the king, and does get to keep his lands, even though his part of the country was involved in a revolution, and most other nobles whose parents were involved in the rebellion got their land taken away. However, in an subversion, the king is just as much of a greedy bastard as ever, and doesn't even dream of giving independence to Garranon's homeland. Even though trying to invoke this trope is the only reason why Garranon consents to have sex with him in the first place.
- Used in The Epic of Gilgamesh, making this Older Than Feudalism. (Except in this case, it's less "Good vs. Evil" and more "Order vs. Chaos.") Enkidu starts off as a wild man, and then he has sex with Shamhat, a temple prostitute over the course of a week. By the end of the week, he is civilized. (It's not clear whether this was a direct result of the sex, or whether Shamhat used sex as a reward for appropriate behavior, thus "training" Enkidu.)
- Happens to half the women Golan Trevize meets in Isaac Asimov's Foundation series, two examples being Hiroko in Alpha Centauri and the transport minister in Comporellon.
- This occurs at least twice in the Incarnations of Immortality series. Incidentally, the series also includes a Sex Heel Turn. However, the Go Seduce My Archnemesis plan worked a little too well, and the newly minted Anti-Villain comes out on top of the ensuing Evil vs. Evil struggle.
- Tahiri Veila in Legacy of the Force. Subverted in that she doesn't actually have sex with Ben. Oh, and Ben's 14 and she's 28, making this an example of Squick. Of course, much Fanon says his father did the same to his mother, making this trope (and just about everything else a Skywalker does) In the Blood. Also subverted, in that instead of the 'sex face turn' working, he converts her later on.
- In Kushiel's Dart two different characters Quincel de Mortbahn and Eamonn of the Dalrida serve as major obstacles to Phèdre's quest to journey to Alba and help Drustan regain his throne so he can bring aid to Ysandre and are persuaded to become more helpful with the promise of a night with Phèdre. In the sequel, Kushiel's Chosen, we have Severio Stregazza, who's a complete Jerkass and generally unpleasant to everyone at court—again, until he becomes Phèdre's first patron, at which point he becomes a pleasant, kind, and helpful ally.
- An Invoked Trope in Do Androids Dream of Electric Sheep?. Androids have been seducing the bounty hunters hunting them to create empathy and thus a reluctance to kill them.
- Kind of played with in Angel at the end of Season 3, where Lilah tries to recruit the abandoned Wesley, even claiming he shouldn't pretend he is too good to work for Wolfram & Hart since he betrayed Team Angel. They end up having sex and develop very quirky feelings for each other, even to the degree of relationship-like behaviour. Well, she was rather turned Face than Wesley Heel. What are the odds ...
- On Buffy, the opposite happened. Angel literally lost his soul and turned evil when he had sex with Buffy, because of a moment of true happiness. Granted, he was kinda cursed to that fate. Spike plays the trope straight-ish, though.
- Season 1 of Battlestar Galactica has Caprica-Sharon pulling one of these after a night with Helo. Ironically, Boomer's sex with Helo, in season 4, only sets up her Moral Event Horizon. A meta-subversion of this trope occurs in the opening moments of the Season 3 premiere: Ellen Tigh is seen sleeping with Cavil, to release Saul Tigh from Detention. Subversion because Cavil ain't one of the good guys and meta because while we can presume this is not her first selfless act, it is the first one the audience sees, and so serves as an inadvertent representation of our increased sympathy for her.
- Daredevil: In the midst of learning about Matt's nighttime activities, Foggy has a one-night stand with ex-girlfriend Marci Stahl. By the end of the season 1 finale, Foggy reminds Marci of what he said to her in her first episode that "she used to have a soul," and convinces her to help Nelson & Murdock dismantle Wilson Fisk's organization. Subsequently, Foggy and Marci's relationship rebuilds itself. They are both working at Hogarth, Chao & Benowitz by the end of season 2, they are back to full-time dating by the start of TheDefenders, and by the start of Daredevil season 3, Marci is more or less the fourth member of the Avocados at Law.
- Game of Thrones. Wildling girl Ygritte works out that Jon Snow is a Fake Defector, but hopes to turn him to their side via this trope. Unfortunately Jon is an Honor Before Reason type, so it doesn't end well.
- Nearly happened in season four of Gossip Girl. Juliet's main goal is to destroy Serena, but begins to turn away from that plan after she jumps into bed with Nate. Then Vanessa convinces her to go back to the dark side.
- On Hercules: The Legendary Journeys however, everyone the Hercster slept with went from unpleasant or downright evil to virtuous and demure. His strength was apparently not his only divine ability.
- Averted in season 3 of Heroes. The viewers are led to believe this is going to happen when it looks like Sylar is falling for Elle. (Admittedly, Elle isn't exactly a shining paragon of White Morality, but any influence she can have at this point can't help but be for the better.) They end up on a beach alone at night, and just as Elle and the viewers think he's going to tell her how she saved him and they're going to sleep together... Sylar whispers that they're both "damaged goods" and can never change, and then he brutally murders her.
- House, the season 6 premiere. Kind of. He's still a sociopath afterward, but at least he's not a crazy sociopath.
- Star Trek: The Next Generation had the Face Turn precede the Sex part. In one episode, Riker is stuck on an alien world which has yet to achieve warp drive (but is very close). He ends up being hospitalized then held captive when they discover he's not the same species. One nurse offers to help him escape... if he'll sleep with her. Cut to her helping Riker escape.
- On an episode of Wings, after Brian reports the improbable-but-true circumstances of his house burning down, a tough insurance inspector arrives to investigate him for fraud. She mercilessly builds her case against Brian, refusing to even consider his innocence... until she and Antonio have a wild passionate hook-up, and in the course of doing so accidentally start a fire that's comedically similar to the circumstances around Brian's house fire. The episode ends with the inspector unceremoniously dropping the case.
- Cruelly subverted in American Gothic (1995), in which Dr. Billy's attempt to seduce Lucas's girlfriend Selena to the path of good ends up with her trying to kill Lucas for entirely selfish reasons and framing Billy for it. This was the result of Writer Revolt on the part of the entire writing team, who had been forced to create Billy by Executive Meddling demands for a conventionally macho hero to be introduced, and took it out on the character.
- In The Epic of Gilgamesh, Enkidu starts off as wild and uncivilized, but then has sex with a temple prostitute named Shamhat over the course of a week. (Perhaps she was using sex as a reward for desirable behavior on Enkidu's part?) By the end of the week, Enkidu is civilized, and no longer roams around with animal companions in the forest, but rather befriends Gilgamesh. Enkidu does blame Shamhat for his death and curses her, however, because if it weren't for her, he would have remained a wild man and not gotten caught up in all the mess he and Gilgamesh did. The sun god reminds him that he is better off because of her, and Enkidu takes back the curse he put on her.
- Can be done in Liberal Crime Squad: high seduction can convert CIA agents, and even the corporate CEO.
- Eva had simple orders in Metal Gear Solid 3: Snake Eater: seduce Naked Snake, steal the Philosopher's Legacy from him, and kill him. In the end, she does steal the Legacy, but leaves him alive with a recording explaining her involvement, and her connection to The Boss. They meet again with much plot relevance before Metal Gear Solid 4: Guns of the Patriots. Possibly subverted when Eva claims that it wasn't her feelings for Snake that stopped her from killing him, but the respect she had for the Boss, who made Eva promise she would let him live.
- Possibly gender inverted in the Neverwinter Nights mod A Dance with Rogues, where having sex with Vico in the second chapter leads you down a path in which he is a somewhat nicer person than your other options in that scene. Played straight in The Bastard of Kosigan, in the first module with the female mercenary in the forest north of Cologne. Having sex with her is an optional part of her HeelFace Turn, though.
- Early in Sengoku Rance, Suzume is dispatched by Inukai of Iga to seduce and kill Rance. She chooses to defect to Oda after having sex with him. Since when is Rance a good guy? When the plot calls for it. Relatively speaking.