Pity Sex

Alice is not normally attracted to Bob, but because she feels sorry for him, she sleeps with him. May or may not be played for laughs. More likely to happen if the one who takes pity is an Ethical Slut. Could lead to a Friends with Benefits situation. Another version of the trope plays out when a prostitute is hired out of similar motives (pity sex by proxy). The G-rated version would be the Pity Makeout.

Usually establishes the one being taken pity on as something of a loser, while portraying the one taking pity in a positive light (especially in the by-proxy examples). Usually tends to be a once-off event rather than an on-going relationship, so do not expect a Relationship Upgrade to take place. Can also be used to establish the sexual mores of the characters by showing pity as enough of a reason to have sex, so one of the characters could be an Ethical Slut, Good Bad Girl or a Chivalrous Pervert. If this is something that is normal in a society, or not out of place, maybe the setting is a Free-Love Future. Might be prefaced by someone invoking You Need to Get Laid, especially in the by proxy version of the trope.

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

Examples of Pity Sex include:

Straight Examples

"Straight" as in "the character taking pity doesn't act by proxy", not "trope played straight" or "not bi or gay".

Comic Books

In Empire State, Sara describes a date with a very unattractive man that ended with the two of them making out, partly because she felt sorry for him and partly because she didn't want him to think she was shallow.

Sara: But I felt so bad for him, Jimmy. He looked so sad. I guess I just didn't want to treat him the same way every other woman does. ... Long story short, Brent calls the next day for a second date, but my guilt had been assuaged by that point so I broke it off.
Jimmy: What a tease.
Sara: Whatever, dude. That make-out session probably got him through the rest of his thirties. He should be happy overall.

Literature

In the Vorkosigan Saga, Miles spends a year on Beta Colony at age 15. He has hopes for unlimited possibilities for sex, but finds that half of those interested (not many) are good Samaritans, and found that he "did not care to be an object of charity".
In Ciem: Vigilante Centipede, Candi finally sleeps with Jack to shut him up so he'll stop annoying her. She sees him as a lovable loser; and takes pity on him for being so obsessed with her when he had more appropriate choices of women all around him the entire time. When it's revealed that Donte is still alive, he astonishes her by revealing that he will not knowingly steal her from Donte. Which shames her into allowing him to have more sex with her, at least until she can reclaim Donte. Ends with Candi finally respecting Jack, Candi finally back together with a confused-but-grateful Donte, and Jack finally able to give someone else a chance.
Averted in Stranger in a Strange Land by Robert A. Heinlein. Jubal Harshaw believes that any of his three secretaries would probably have sex with him out of pity, but he himself is too proud to accept such charity.

Live-Action TV

In Community, Professor Slater first agreed to sleep with Jeff because he made himself seem pathetic, begging and pleading enough until she finally said yes. Subverted in that it never happened in that episode, and instead they first slept together after dating conventionally.
Subverted in All My Children. This was Krystal's excuse for sleeping with Tad- emphasis on excuse, since she did have feelings for him.
On Reno 911!, Dangle threw Trudy a "pity fuck". Jonesy did the same for Dangle once, too.
Happens on the first episode of Friends With Benefits after Aaron gets dumped and Riley feels sorry for him.
In Soap Jody has turned Carol down multiple times due to his homosexuality. When they're on holiday together, as friends, she starts crying and as he comforts her, they end up having sex.
Better Off Ted: When Veronica was promoted to management, there was another employee with the same last name up for the same promotion, and a typo in the memo made it unclear which of them had been promoted. A few years later she meets the other employee again and finds out that his life since then had gone completely down the toilet, so she starts dating him out of guilt. She finally brings herself to dump him when Ted finds another memo proving that the promotion really was meant for her.

Web Comics

In Ménage à 3, DiDi wants to "deflower" her friend, Gary, who is a hopeless virgin. Various mishaps prevent her from doing so; later, Zii explains to her that she shouldn't do that, because Gary would associate sex with being pathetic. (And because she wants Didi for herself, but of course, she didn't tell that to her).
Dork Tower tries to put a positive spin on this, in this strip drawn immediately after the Alyssa Bereznak debacle.

Matt: Dude is gonna get more sympathy nookie from cute geek girls than any human has the right to have.
Ken: He's been dealt the Beta Black Lotus of canoodling, yes ...

Western Animation

Lana has sex with Pam after taking pity on her in the first season finale of Archer.
Leela and Zapp on Futurama, and there's a suggestion that he uses this intentionally to make women sleep with him.
In Drawn Together Xandir, who is Camp Gay, "tosses a mercy-fuck" to Hollywood Pudgy Toot. He clearly didn't enjoy it.
On American Dad, Hayley needs a favor from her brother Steve. The price is admitting that Steve might one day get a long-time female friend to have "awkward pity sex" with him once.

By Proxy Examples

Literature

Subverted (or maybe zig-zagged) in A Song of Ice and Fire. Tyrion's first sexual experience is with a woman who seems to just love him for who he is. Then it turned out that his brother Jaime hired a prostitute to play the part of Tyrion's loving girlfriend. Then it turned out that that was a lie Tywin forced Jaime to tell Tyrion -- the woman wasn't a prostitute and really did just love Tyrion for himself.
In Devdas after learning that he cannot be with Paro anymore, the titular character's friend introduces him to a High-Class Call Girl named Chandramukhi, in an attempt to cheer him up.

Live-Action TV

The pity sex by proxy happens in Boston Public; a romantically starved character discovers that his hot new girlfriend who actually seems to get him is in fact a prostitute hired by his wealthy brother. He didn't react well. (The 'girlfriend' was under the impression that the man already knew that he was getting "the girlfriend experience" and broke character at the wrong time.)
The other cops hire a hooker for Renko's birthday in Hill Street Blues, and he was pissed when he caught on.
In The Big Bang Theory, Leonard and Raj hire a prostitute for Howard on a trip to Las Vegas that happens right after Leslie Winkle breaks up with him, though any sex that might ensue is glossed over.