Sex with the Ex

Two characters that used to be a couple broke up. They got bored of each other or argued too much, he just wouldn't put the seat down, she used too much toilet paper, whatever. Emotionally or mentally they didn't think they were right for each other. Sexually, however, they still get along just fine.

They may only get to kissing each other or get further than that, but as long as they're reinitiating a physical relationship it's fair game. It can naturally take on a variety of forms, as people are complicated; it may just be sex and friendship, it may be a pattern of hooking up, swearing to never see each other again, and hooking up again, or they could go back into a relationship.

May lead to getting back together, particularly if it starts after a separation or after they've moved on to other people for added drama/comedy. Naturally, a common sitcom trope.

Subtrope of Amicable Exes, where they're really amiable. Supertrope to New Old Flame.

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

Examples of Sex with the Ex include:

Anime and Manga

Lupin and Fujiko engage in this, due to the nature of their on-again off-again relationship. The most notable instance occurs in The Secret of Twilight Gemini. They go at it twice, during one of their off periods, despite Lupin's interest in Lara. The first attempt gets averted by Sadachiyo and his men, who crash their hotel room to assassinate Lupin. The second attempt, however, goes off without a hitch. He bangs her in the file room of a Moroccan police station!!

Comic Books

Silver Sable did so at least once with her ex-husband The Foreigner, leading to a brief pregnancy scare. (She wasn't.)

Film

In The Wedding Singer, Adam Sandler's ex-girlfriend takes advantage of his drunken blackout to have a one-night stand with him. He rejects her once he sobers up the next morning.
Happens between the title character and his ex-wife in The Lincoln Lawyer.

Jokes

There is an old joke about a woman who visits a friend she didn't see for years, and sees several children who look just like the husband their mother divorced years ago. When questioned, said mother says "He sometimes visits in order to apologise".

Literature

The Book of Joe has Joe and his former high school girlfriend Carly reconnect when he revisits town. It is implied that they'll rekindle their relationship following their night together.
In Suzanne Finnamore's memoir 'Split,' she admits to having sex with her ex in the midst of their messy divorce, even though he's theoretically with The Other Woman (the same one he left Suzanne for) and she claims that sex is The Best. Ever.

Live-Action TV

Used as a plot device in the Pilot Episode of Cheers, where Diane's fiance jilts her so he can run off on their planned honeymoon with his ex-wife, thus forcing her to work at Sam Malone's bar for several years' worth of UST.
Friends: Ross and Rachel were guilty of this, several times, during break-ups, and when Monica and Richard break up the second time she offers him one last night together.
The Golden Girls had an entire arc of it with Dorothy and Stan, leading up to the intended, but called off, re-marriage.
Used to definitely break up Rachel's relationship at the end of the first season of How to Make It in America. She ponders over breaking up with her boyfriend, gets drunk, sleeps with her ex Ben, then tells her current boyfriend it's over and underlines how over it is by mentioning she slept with Ben.
Scrubs: Dr. Cox started sleeping with his ex-wife Jordan, who is one of the few people with a strong enough personality to keep up with him. After about a season and a half of this just loathing himself and sabotaging any attempt at a new relationship, they get back together officially when Jordan becomes pregnant and remain together officially unmarried the rest of the series.
My Name Is Earl: Earl's wife Joy divorced him in the first episode and those early episode had a very rocky relationship. When Earl accidentally ruined Joy's marriage to Darnell he set out to make it up to her, which managed to get them back together as friends and accidentally sparked a night together, forcing Earl to need to make things up to Darnell.
The Big Bang Theory: Leonard and Penny broke up late-third season and in the season finale Penny started dating again, only to find herself unable to go back to stupid guys after dating Leonard (a physicist). Coming home drunk and depressed she took Leonard to bed, which he misinterpreted as Penny wanting to get back together. She was very embarrassed because that was not the intention (she gets very loose when she drinks) and Leonard was upset over being used for sex.
Castle and his second ex-wife briefly remembered why they got married and spent the summer in the Hamptons together - which reminded them why they got divorced.
How I Met Your Mother had an episode revolving around Ted and Robin having sex in order to avoid arguments (they had broken up but had recently become roommates because Robin lost her job). Interestingly enough, their activities did not end because of romantic complications, but because Barney was secretly in love with Robin at the time, and when Ted figured this out (due to Barney's not-so-subtle attempts to stop them from fighting and therefore having sex) he immediately called off his arrangement with Robin and allowed her to believe it was because he was afraid he'd redevelop feelings for her.
Frasier does it at least once, with his ex Lillith Sternin.
In one episode, Blossom catches her divorced parents fooling around. They hadn't had sex but may have if they weren't caught.
Seinfeld did this, with Jerry and Elaine.
Cougar Town: Jules and Bobby hooked up after having been divorced for a while. Grayson and Andy wrote a song about it, which they called (appropriately), "Sex with Your Ex." It went:

Sex with your ex is really great/but sex with your ex is a big mistake/oh no no/sex with your ex.

Happened in Pan Am when Bridgett came back; screwed up Dean and Collette's relationship.
The fourth season of Breaking Bad has Walter having sex with his wife again after they split.
Raylan Givens in Justified has slept with his ex-wife several times, even though she's now married to another man.
Bones: Booth has had a couple of "one time only" sleepovers with his ex-girlfriend, the mother of his son. Similarly Bones herself slept with her former boyfriend (and thesis supervisor) when he came into town.
Parenthood - Crosby and Jasmine end up in bed together after discussing (over copious amounts of wine) the heartbreak they both felt trying to explain to their son, Jabar, that they wouldn't be getting married.
In That '70s Show episode "The Relapse" after her mother leaves, Donna is distraught and ends up having sex with her ex-boyfriend, Eric. Eric believes that this means that they're back together, and is quite upset when he learns that it doesn't.
House of Lies opens with Marty regretting a night spent with his ex-wife. Doesn't stop him from doing it again in other episodes.

Video Games

In Fahrenheit (2005 video game), Lucas can bed his ex-GF Tiffany when she comes to pick up the last bits of her stuff from his apartment mid-game, complete with an interactive sex scene (that was eventually cut from the American release).
Silent Hill: Shattered Memories: Harry is talked into having sex with Dahlia (who claims to be his wife, and that they've done it plenty of times before). Of course, given The Reveal, it's doubtful that this has ever actually happened.