Sex Face Turn

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 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 to 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 Heel Face Turn and In Love with the Mark. Very common as a result of Go Seduce My Arch-Nemesis, 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.

For a darker relative, see Rape as Redemption. Related to Sex Is Good.

No real life examples, please; this is All The Tropes, not Tropes After Dark.

Examples of Sex Face Turn include:

Anime and Manga

Okiku from Afro Samurai.
This is possibly subverted/played around in Gun X Sword. Michael Garret performs a Face Heel 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 Heel Face 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 Heel Face Turn there (provided that certain steps are taken).
Inverted in The Girl in the Library: Until The Pure You Falls From Grace where Yukiha goes from a sweet good-natured girl into an evil slut when she becomes her rapist's Sex Slave willingly after being left unsatisfied by her boyfriend's small penis and lack of experience.

Comic Books

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.

Fan Works

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".

Film

Averted with Loretta Salino 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.
Eve in North by Northwest, although technically since she was a mole all along she never really loved her crime boss partner in the first place.
Lili Von Shtupp from the movie 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.
Austin Powers: The Spy Who Shagged Me. Hallota Fagina and Austin Powers.
Undercover Brother. White She Devil and the title character.
James Bond examples:
- Pussy Galore in Goldfinger.
  - 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). After what she says happened to her at the end of the novel, I think that'd be enough to put any girl off men to some degree.
- Tiffany Case in Diamonds Are Forever.
- 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!

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. Symbolism much?
Subverted with Mayday in A View to a Kill. While she does sleep with Bond, that is not what causes her to switch sides - that occurs due to Mistreatment-Induced Betrayal on the part of her employer.

Gila in Our Man Flint, with Derek Flint.
Meg from |Hercules, although as far as we can tell it wasn't actual sex that did it.
Sorsha from Willow with Madmartigan.
Achilles in the film Troy with Briseis. He gains a heart and purpose from there on.
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 her in the Moon Room during the Carnival.

Lewis: Jocks only think about sports. We [nerds] only think about sex.

In The Pink Panther Strikes Again, the same effect is achieved not by the antagonistic woman actually seducing the hero, 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 the hero very confused when she suddenly appears naked in his bed professing her undying love for him.
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?"

Literature

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 Arch-Nemesis plan worked a little too well, and the newly minted Anti-Villain comes out on top of the ensuing Evil Versus 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.

Live Action TV

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.
- Well, Xena: Warrior Princess went from mass murderer to cranky defender of the innocent after sleeping with him once. Still a pretty impressive batting average for Herc. After all, All Amazons Want Hercules.
  - Of course he also possibly turned her lesbian. Or at least Bi.
- In classical mythology, the dude got around so much that it's astonishing that Greece didn't turn into a fantastic utopia. Except for the whole thing with him going loco and killing them and all.
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, I'd say 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. The converse also happened, with Spike. One could say this trope was subverted, when Spike decided to get a soul because of his love for Buffy.
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.
House, the season 6 premiere. Kind of. He's still a sociopath afterward, but at least he's not a crazy sociopath.
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.
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.
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.

Video Games

Eva had simple orders in Metal Gear Solid 3: 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.
- 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 Heel Face 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.
Can be done in Liberal Crime Squad: high seduction can convert CIA agents, and even the corporate COE.

Web Original

Pretty much the entire premise of Pokegirls.
The Evil Overlord List, #19. Stick to the list and you shall succeed.
Leather from Interviewing Leather, has some really nasty things to say about "heroes" who think they can redeem a supervillainess by sleeping with her.