A sexy situation outside a character's relationship causes them to seek fun with their partner.
Bob gets aroused by Cassie, but acts on it with his committed partner Alice. In many works, for a partnered person to be aroused by any Lust Object besides their partner is a forerunner to danger, or at least high drama. But sometimes a character will perform a kind of lust judo and redirect their heightened libido in a more acceptable direction. The situation need not be an actual encounter with a person, it could for instance be hearing a story, including one involving their partner but not themselves. However, things being as they are, the math is often that a man hearing about his female partner's experience with a woman will lead to retargeted lust, as will a woman hearing about her male partner's early experience with another woman, but other combinations will just lead to trouble. Given that many partnered people live their lives surrounded by potentially arousing people, images, and ideas, and yet somehow most of them remain mostly monogamous most of the time, it is probable that this phenomenon is if anything more common in Real Life than in fiction. Does not include circumstances such as a couple watching pornography together, which is more passive, planned and "safe" and should probably be its own trope. Also does not include cases where a character tries to use lust for one person as a way of overcoming their Incompatible Orientation from their partner, which is a form of Gayngst. If the Lust in question leads to solitary activity (or at least that without another living being), then it's more of an I'll Be In My Bunk moment.
- In It's a Wonderful Life, Bert the cop, after watching the slinky Violet walk down the street, says: "Think I'll go home and see what the wife's doing."
- In Girl with a Pearl Earring, after the married artist and his model have an unplanned steamy moment, they separate. She goes off to have her way with the butcher's boy who always liked her, while he has sex with his wife.
- In The Tailor of Panama, a married woman has tense and increasingly sexy banter with the spy who may be endangering her husband. After he leaves, she has sex with her husband and he with his new girlfriend.
- In Wild at Heart, just hearing about one of her lover's past encounters makes a young woman, in her words, "hotter than Georgia asphalt."
- It seems like the wife in Eyes Wide Shut was partly trying to get this reaction from her husband when she tells him her fantasies about running off with a sailor, but instead it plunges him into turmoil and doubt. So she eventually takes the direct approach.
- Kinda what happens in "Victims of Circumstances" novella from the first volume of Labyrinths of Echo: Max and Melamori, who kept having rather moist dreams about each other (despite not really being in a relationship when awake), fall prey to some Wild Magic and wake up in the same bed one morning. After all the yelling and name-calling, Melamori storms off, saying she'll go visit the local Red Light District and have sex with a total stranger, just to spite him. Before long, he ends up in said district, as well. Because the "brothels" in Echo work differently from our world (it's more of a blind dating than prostitution), they end up being accidentally matched together, anyway—which only complicates their relationship even more.
- In the first episode of Coupling, a woman doles out (possibly fictitious) stories of her past encounters with women in order to keep her boyfriend interested.
- In Married... with Children, it happens several times that Al, turned on by other women, has sex with Peggy (something he normally detests). In the episode, "L Ive Nude Peg", the "other woman" is actually Peggy, who performs a striptease in Al's favorite strip club wearing a veil.
- On The Big Bang Theory, Howard expresses arousal at some girl in a gym they're visiting; Penny is disgusted, so Howard explains that Bernadette 'doesn't care who gets my engine running, as long as I park it in the right garage'. Penny can't believe he's with Bernadette.
- An episode of Roseanne in which Dan meets his old high-school girlfriend, then goes home and has amazing sex with Roseanne. When Roseanne works out the chain of events, she gets jealous.
- South Park: A bout of impotence keeps Gerald Bartofski from having an erection until a couple of women come into his office to complain about global warming which is making them get skin cancer. They take off their clothes to show him, he gets exited, and runs home to have sex with his wife.
- TheSimpsons: In the episode "Feelin' Frisky", Homer hints at a heightened desire for Marge after watching women's volleyball on ESPN.
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