Sex by Proxy

So, Alice and Bob are making the two-backed beast, so to speak. Except it's more of a three-backed beast. With or without them being aware of it, a third party is actively enjoying their encounter, and not in the usual way, either.

This trope is when:

Someone is telepathically or otherwise linked to the characters, and can feel whatever they're feeling;
Someone is participating in the rompin', but isn't physically there;
Someone experiences the effects of sex long after it takes place;
Someone's mind is magically transferred into the body of one of the characters, with the body's actual owner still conscious.

Please don't list examples of body snatchings that lead to sex (with the body's original owner left unaware), or examples of characters being involved in sex without realizing it (hey, it happens). Those are both different tropes.

This tends to be a magic trope, although sci-fi versions exist as well. Often an upgrade of Dirty Mind Reading. It's Fetish Fuel, obviously, and a subtrope of Power Perversion Potential.

No real life examples, please; at least, not until we have evidence that supernatural abilities exist.

Examples of Sex by Proxy include:

Anime and Manga

In Ghost in The Shell (the manga), the Major participates in porn that allows the viewer to feel whatever she's feeling. She accidentally gets mentally linked to Batou, who feels just icky because he doesn't have the organs being stimulated. Predictably, the scene was censored in the US version, but present in European editions.
In Pet Shop of Horrors, Leon is seduced by two different women in one night. It turns out that they're plant spirits who were using him to spread their pollen from one girl to another.

Comic Books

In Elf Quest, when Cutter realizes that Zhantee has always been in love with Leetah from afar, he mentally shares his memories of his relationship with Leetah with Zhantee as a gift - including their romantic intimacy.
In Requiem Vampire Knight, there is a character capable of stealing orgasms from others as they are about to happen. "The ultimate safe sex" indeed.
This seems to be at least a significant chunk of the principle behind the Shrieky Girls of Doktor Sleepless.

Fan Works

A more PG-rated version occurs in Kyon: Big Damn Hero. Due to being in tune with Haruhi's emotions at all times, especially if the emotions are particularly strong, Koizumi ends up extremely flushed when Haruhi and Kyon are making out.
Multiple Firefly fanfics have explored the implications of River's long-established Power Incontinence while stuck aboard a very small spaceship with her big brother right after he finally scores with Kaylee.

Film

In Ghost, Sam uses Oda Mae's body to say goodbye to Molly one last time. Although it's more of a PG-rated Makeout By Proxy.
In Being John Malkovich, Maxine has sex with Malkovich while Lotte is inside his head. She doesn't care all that much for Malkovich - she just gets off on "Lotte in Malkovich". Malkovich slowly becomes aware of what's going on. Things get out of hand.
In Freaks, one of the conjoined twins is shown to get aroused when her sister is kissing her boyfriend. Since the sister and boyfriend proceed to get married and have sex, one can assume the trope is in full effect.
- In the musical Side Show, which is based on the lives of the same conjoined twins, there's a scene where the girls and their respective love interests are on a Tunnel of Love ride. Violet seems to be able to feel Daisy's reaction to being kissed by Terry (and notes that it's much stronger than her own response to being kissed by her fiancé).
Strange Days involves illegal devices which can record and play back memories - giving the user all of the sensory input felt by whoever was being recorded. The recording include sex, of course (parts of a couple of raunchy memory recordings are shown, both for sale and for personal use). It's probably a huge part of the market.
In Surrogates some people willingly let others access their robotic avatars to feel the pleasure of living in a perfect body (and touching it).

Literature

In Robert A. Heinlein's Stranger in a Strange Land, when Jubal finally has sex with Jill and/or Dawn, he discovers that the entire cult has enjoyed their romp through their shared mental link.
In the Wheel of Time, the Warder bond allows people to feel what others do. So when Elayne and Rand finally have sex, Birgitte (who is bonded to Elayne), Min, and Aviendha (who are bonded to Rand) all have to go off and get drunk so they can't feel precisely what is going on.
Walter Jon Williams' Aristoi, which is set in a future where everybody has wireless internet connections in their heads, has a science-fictional example.
In the Dragonriders of Pern setting, the mental link between dragons and riders can cause this effect. Generally, when dragons mate, their riders are pulled into the lust and wind up following suit whether they want to or not - but it also works in reverse, most noteably in the case of Lord Jaxom, who winds up with a strange, white runt of a dragon. Being apparently asexual, it has no interest in 'flying' with female dragons, but it DOES seem to rather enjoy sharing Jaxom's feelings when HE'S off with a girl.
In the novel Geek Love, conjoined twins Elly and Iphy have sex with a man. Elly wants it, and Iphy... doesn't. It gets even more squicky because the two share the same bottom half.
In The Dresden Files novel White Night, resident wizard gumshoe Harry and his apprentice Molly are trying to get an impression on the last moments of a woman who has mysteriously died. Because of Molly's talents with magical detection, she's the one who'd tasked with picking out the dead girl's last sensations. It turns out, her last moments were wrapped up in the lethal sexual ecstasy of a White Court vampire, and Molly catches the full blast.
At one point in the Sword of Truth series, Nicci links herself to Kahlan and then lets a violent brute have sex with her, forcing Kahlan to feel every sensation as if she were the one getting it.
In Commander-in-Chief by Nikolai Gudanets, a psychic alien squid mind-controls a Call Girl and arranges for her to be ordered by an American government official. After taking control of him as well, the alien realizes that the man is still aroused, so he has them do it, while connected to both. This leads to a rather graphic scene in which the alien has trouble with pronouns due to being in both minds at the same time. It even experiences orgasm, a strange feeling for its species. The second time it happens turns deadly, when the official accidentally bites a hooker's jugular during sex.
In the Heralds of Valdemar series, Talia has a permanent empathic link with her Companion that can't be blocked, even when her Companion is busy showing the Companion mares what a virile stallion he is. As The Empath, she's also treated to the emotions of her fellow Heralds whenever they're nearby unless she very carefully shields herself. Being initially roomed next to an exceptionally randy trainee, it is no surprise that she later chooses to move her permanent quarters to a remote tower of the Collegium.
Animorphs doesn't give us all the details (it is a kid's book, supposedly), but Visser One and one of her first human hosts did this with another human-Controller, resulting in a pair of twins with four "parents." And of course, other Yeerks probably wind up doing this quite a bit too.
In Marge Piercy's Woman on the Edge of Time, the main character travels to the future and has sex with someone she meets there. It's only after the fact that she learns that the mechanism behind her time travel (which relies on a psychic bond between her and another person from the future,) meant that said other person was aware of the sex as it was going on.
In the Night Angel Trilogy, when Kylar has sex with Elene, it is picked up by Vi, who is telepathically linked to Kylar through magic earrings. As she was also in love with Kylar, she enjoys it, but feels dirty as well as she views it as too voyeuristic.
In the Lilith's Brood series, the Oankali are a three-gendered race, and perpetuate that manner of relationship when they mix with humans. It roughlyly involves the ooloi being in-between the male and female and connecting their minds, so that they have sex mentally while never touching each other, only the ooloi conduit, and the ooloi enjoys all the feelings of both.

Live-Action TV

On Firefly, River (a telepath) once walked by as Wash and Zoe were making out in the cockpit. She is able to feel what both of them are feeling and visibly reacts.
When Phoebe developed her empathy powers in Charmed, she became capable of feeling the emotions of people around her. This led to a fairly Squick implication of this trope when she was walking with Piper (her sister) and a guy was checking Piper out. Upon beginning to feel his attraction towards her own sister, Phoebe informed Piper of the fact, and the two decided to quickly move right along.
Telepaths in Babylon 5 already have problems keeping everyone else's thoughts out, so guess what happens when two powerful telepaths get it on in the middle of a telepath colony.
Fringe: Olivia gets this in the episode "Bad Dreams", as part of an ongoing psychic link with the episode's killer.

Tabletop Games

Traveller: There is one alien race in Traveller where all births are in mindlinked brother-sister pairs. These are each linked so closely that they are almost one individual. When they marry one pair marries another rather then one individual marrying another. Sex by Proxy is an unstated but obvious inference.

Video Games

In Dragon Age II, this is brought up if Hawke romances Anders (the latter of whom is possessed by a spirit of Justice). When Anders tells Hawke that Justice disapproves of their relationship, Hawke can ask, "So, he's like an unwilling participant in our threesome?". Later, Isabela makes the same joke, commenting, "You, Anders... and Justice? Well, you know what they say - two's company, but three's better."

Web Comics

A romantic version (and, presumably, at some point, a sexual one) occurred in A Miracle of Science, when Mars falls in love. Prester meets a cute, female Martian, who is part of the Martian Hive-Mind, and they get along well... to say the least. When he then visits Mars, he finds that random people keep giving him flowers and smiling at him - because, after all, Mars LIKES him. (The series ends before their relationship can progress to the final level of intimacy, but one must imagine that it'll be SOMEWHAT awkward...)