Rape by Proxy

Two people are coerced into sex by a third party. This can be part of a Sadistic Choice—wherein the villain will threaten one or both of the victims with bodily harm, unless the two cooperate—or it can be under pure coercion from one who is more powerful or manipulative.

This is handy for a villain who doesn't like to get his hands dirty. It also lets him use his victims against each other—making it very efficient.

An extra dimension of angst can be added when two characters have deep feelings for each other, but those feelings are not sexual in nature (e.g., friendship or platonic love); alternatively, they may have latent romantic feelings, but exhibition is not how they would have wanted to express it. Judging by the examples below, it's also disturbingly common for the pairing to be incestuous.

It can be heartwrenching, especially when the victims struggle to comfort each other in the midst of the mutually traumatic ordeal, or suffer the perversion of their honest affection. It can also be an Excuse Plot for pointless titillation.

Compare/Contrast Sex by Proxy. One might expect Aliens Made Them Do It to be the Sci Fi counterpart, but in fact that trope is much Lighter and Softer, often played for laughs or a light-hearted Ship Tease.

Anime/Manga

 * In the (adult) manga series Aki Sora, Sora's friend Miharu coerces Sora and his girlfriend Alice into have sex (more than once), because he is not able to do so. Neither one takes much convincing, and it is played for laughs.
 * Robotech has an episode where Rick, Lisa, and Ben are captured by the Zentraedi and ordered to demonstrate human procreation. Because the Zentraedi are completely ignorant about sex and are hugely disgusted by acts of intimacy, Rick and Lisa manage to get by with just a forced kiss instead of forced sex.
 * In Night Shift Nurses Ryuji forces Hikaru and Remi to have sex since he has blackmail material on both of them.

Comics

 * In Irredeemable, the Plutonian is shown forcing a man and woman to make love while roleplaying as him and a former lover.
 * In a controversial story, a villain called Sleez was heavily implied to have forced Superman and Big Barda to make a pornographic film using Mind Control. Word of God has said that nothing was confirmed, so the truth is up to the readers.
 * Happens in Fables when Bluebeard uses magic on both Snow and Bigby to make them leave town and spend a week in the woods together, where they have sex and don't remember it.

Fan Fics

 * In Hinge of Fate by ramos, Snape and Hermione are forced to do this when the latter is captured by the Death Eaters. It's presented as the lesser of two evils, with Snape being the best choice to perform the act. It is implied others would not be as gentle.
 * A very thorny case crops up in the Tron: Legacy Dark Fic "Obsidian." Clu hands over a "repurposed" Yori as a "prize" for his enforcer, Rinzler. Now, remember Taking the cake?
 * This Star Trek: Enterprise fanfic, pairing Malcolm Reed and Trip Tucker. Notably, the author deals quite well with the problems this creates between the two, as their mutual attraction meant they enjoyed it but the feelings of violation were still there, and it takes them quite some time to untangle the two.
 * In the Pony POV Series, this happened with Filthy "Onyx Tiara" Rich and his household staff, under the influence of Discord. While Filthy was discorded, anypony he touched (even the one male member of the staff) turned into an imitation of his wife, and, well... Yeah. And of course, they all remembered what happened afterwards. It is unfortunate that the goat-faced creep responsible cannot be permanently dealt with.

Film

 * The Butterfly Effect: Kayleigh's father orchestrated this between Evan and Kayleigh...as children.
 * The Last House on the Left: In a moment of poorly expressed empathy, Willow suggests, "Have them do each other!" to divert the villains from the even more vile tortures they were brainstorming.
 * In One Hour Photo, the Villain Protagonist forces this between an unfaithful husband and his lover while he takes pictures.
 * Se7en has an extremely violent and disturbing example. As always in this movie, we see only the aftereffects. It also results in Murder by Proxy.
 * The 2009 Mexican film Daniel and Ana, which falls somewhere between Based on a True Story and The Tasteless But True Story in its graphic nature, concerns a brother and sister whom pornographers kidnap and force at gunpoint to have sex with each other on camera.
 * In the movie Primal Fear, the main character discovers a home video owned by the murdered archbishop, in which the archbishop forced two alter boys to have sex with a girl on camera.

Literature

 * Shards of Honor: Vorrutyer orders Bothari to rape the prisoner of war Cordelia Naismith. This is a common enough order from Vorrutyer to Bothari that Vorrutyer does not expect to be disobeyed.
 * In Mario Puzo's The Family, Rodrigo Borgia forces his son to deflower his daughter, believing that a woman will forever be loyal to the man who takes her virginity.
 * In White Midnight by Dia Calhoun, Mr. Brae wants an heir to his estate, so he forces Rose to choose between sex with him and sex with his son Raymont (who also has no choice in the matter, and is widely rumored to be a hideous monster). She chooses Raymont spoiler: and eventually has his child, but she's not happy about either option. Oh, by the way, Rose is fifteen, and Raymont is maybe seventeen.
 * In A Game of Thrones, it is revealed in back story that Tyrion once married a prostitute, but the marriage was ended when his father Tywin found out about it, and as punishment Tywin made Tyrion watch as his guards raped her, finally forcing Tyrion to go last.
 * It Gets Worse. There is a chance she wasn't a prostitute and Tywin lied.
 * Also, Ramsay Bolton forces "Reek"  to perform oral sex on Ramsay's thirteen-year-old bride.

Live Action TV

 * Spartacus: Blood and Sand: On the whim of a wealthy pervert guest of Batiatus, Gannicus is forced to have sex with his best friend's wife. Being a slave sucks.
 * The murderer in the Criminal Minds episode "Compromising Positions" had this as part of his MO.
 * In season one of Skins, a bunch of guys tried to force Tony to rape his sister (who was unconscious from a drug overdose at the time). They actually weren't planning on making him do it, though, they just wanted to scare him, which they succeeded at quite spectacularly.
 * Law and Order SVU had a case where a boy was born with a twin brother, but the brother's circumcision was botched, so the doctor's 'fixed' it by doing a sex change operation at the parent's request. The parent's raised the child as a girl with the help of a psychiatrist who strongly believed that with the proper Nurture they could overcome Nature. To try and teach the 'girl' to be a girl the doctor used 'her' brother to teach 'her' her part in a physical relationship by putting them into the positions (though there never was penetration), while they were still in Junior High.
 * Used in an episode of Law and Order: SVU featuring Robin Williams, although he didn't graduate to actual rape, even coming off as something of a counterculture Anti-Hero. This was Ripped from the Headlines, based on a long-running scam artist.
 * It Got Worse—the real life guy did ultimately graduate to rape. Bastard.
 * In another SVU episode a pair of spree killers would attack groups of people and force the men to rape the women, and then to rape each other before killing them. Even the seasoned sex crime detectives who deal with "ordinary" rapists and paedophiles virtually every episode thought these killers were worse than what they were used to.
 * A storyline on Nip Tuck had the elderly husband of the woman Christian was currently bedding demanding that they have sex in front of him. He was unable to have sex due to his age and health problems but still wanted to get off on seeing her get off and decided to use her affair with Christian to his advantage.

Web Comics

 * In Dominic Deegan, Stonewater's rape of Melna (as opposed to letting the host tribe execute her) was clearly meant to come off this way. Too bad nobody told them: For years Melna blamed Stonewater and Stonewater blamed himself.
 * Melna eventually wisened up to it in the rebirth of Maltak arc, and killed the guy responsible.

Western Animation

 * In Futurama, the probe "V-Giny" forces Leela and Zap to have sex to reenact Adam and Eve.