Two people are circling each other romantically, or perhaps are already together. Other characters in the story smile fondly at the couple and ship them. And then there's this guy. He does not like the two people together, makes his opinion known, and clearly hopes that the ship will sink. This may be for a variety of reasons:
He's secretly (or not-so-secretly) in love with one of them.
Saeki from Strawberry Shake Sweet is very opposed to Julia and Ran's relationship and continuously tries to sabotage it—ostensibly out of fear of a scandal that will end all three of their careers. That she is the Token Straight Girl in a Cast Full of Gay probably has something to do with it, too...
Lady Tremaine aka the Wicked Stepmother in Cinderella wanted to keep Cinderella away from Prince Charming because she hoped he'd choose one of her daughters, whether it's Anastasia or Drizella. In the third movie, she also objects to Anastasia not only having a Heel-Face Turn... but also falling in love with a baker boy beneath her station. This contrasts with Cindy being a total Shipper on Deck for Anastasia and her boyfriend.
In Tangled, Mother Gothel tries to break Rapunzel's faith in Flynn to keep Rapunzel dependent on her. When her last ditch involves Flynn being severely wounded, she tries to force Rapunzel in a Sadistic Choice to save him. It backfired. Fatally. On HER..
Briefly happens in Anastasia, when Vlad worries about Dmitri and Anya's budding romance. He quickly shifts to a Shipper on Deck, though.
In Emma, most people think that Emma and Frank Churchill belong together. The only one who thinks that Frank doesn't deserve her is Mr Knightley.
Lady Susan: Reginald's whole family is horrified when they find that he's in love with Lady Susan who is a beautiful woman, but evil and rotten to her very core.
In Northanger Abbey, General Tilney heavily ships his son Henry and Catherine Morland because he thinks she's a rich heiress. When he finds out it's not true, he throws her out of their house and forbids Henry to think of her.
Persuasion: Before the novel begins, Anne Elliot and Captain Wentworth got engaged, but Anne broke it off. Her family was against their marriage, especially her dear godmother Lady Russell.
Miss Bingley and Mr Darcy actively break up Jane Bennet and Mr Bingley. She's a snob who ships him with Miss Darcy. Mr Darcy thinks Miss Bennet doesn't love him.
Mr Darcy doesn't like that George Wickham flirts with Elizabeth. He's jealous and he knows that Wickham is not an honest man.
Mrs Gardiner, Elizabeth's aunt, advises against Elizabeth's possible relationship with Wickham because neither of them has independent fortune.
Darcy reveals that he found out that his sister nearly eloped with a man but luckily he stopped them.
Lady Catherine is furious when she hears rumours that her nephew Mr Darcy is love with Elizabeth Bennet.
In Sense and Sensibility, Edward Ferrars' family ships him with wealthy Miss Morton and they are against him courting Elinor Dashwood. Edward is however secretly engaged with much poorer and less genteel woman, which is even worse for them and poor Edward is disinherited.
Scarlett in Gone with the Wind is decidedly this way towards Ashley and Melanie, since she believes Ashley is her true love.
Ron is not a fan of anyonedating his little sister Ginny. It even takes him a little getting used to the idea of his best friend Harry dating her. Ginny eventually tells him off.
Ron vehemently tries to torpedo Hermione/Viktor Krum, although at this point he Cannot Spit It Out as to the real reason why.
In flashbacks, Snape towards Lily and James.
In a more humorous example, this is initially Mrs. Weasley's reaction to Bill and Fleur.
Alec Lightwood of The Mortal Instruments isn't a fan of Jace/Clary at first, since he has feelings for Jace himself. But once he falls in love with Magnus, he calms down about it.
In Team Human Mel is adamantly opposed to her best friend Cathy's romance with Francis, a vampire. This is partly Fantastic Racism and partly honest concern for her friend's well-being. Cathy is 17 and Francis is over 150, and the process of becoming a vampire has the risk of killing Cathy or turning her into a zombie.
Xander was king of this trope: he loathed Buffy with Angel because he's in love with Buffy; was a Torpedo to Spike/Buffy because he dislikes Spike and thinks he's bad for Buffy, and anti-shipped Wesley/Cordelia for all of the above reasons.
Pretty much everyone was against Spike/Buffy- even the two of them on occasion.
Wesley isn't fond of Fred's relationship with Gunn and less so of her flirtation with Knox, and the Squick that accompanies the pairing of Connor with Cordy makes it unpopular in-universe and out.
Lorelai is against Rory/Jess, instead shipping the rival Rory/Dean. Later she also opposes Rory/Dean (since he cheated on his wife) and is against Rory/Logan as well, thanks to Logan bringing Rory back to rich life she ran away from.
Rory is anti Christopher/Lorelai (despite them being her parents, she realizes they're a poor match) and ships the opposing Luke/Lorelai.
Richard and Emily vehemently torpedo Luke/Lorelai and Rory/Dean, to the point of Relationship Sabotage. Emily also dislikes Rory/Jess.
Marty is in love with Rory and thus not a fan of her and Logan
In a humorous example, Taylor initially opposes Luke and Lorelai's relationship out of a fear of what the breakup would do to the town. He even draws up maps for division in event of a breakup.
Heartwarmingly subverted on Castle, when everyone assumes Gates would flip out if she discovered Rick and Kate's relationship. Turns out she's as big a Shipper on Deck as the rest of the cast.
Lily from How I Met Your Mother is this for many of Ted's girlfriends, as he has a habit of thinking horrible women who aren't looking for a meaningful relationship and who invariably dump him are "the one", and Lily winds up actively sabotaging several of his relationships. Ted does not take this revelation well, especially when it slips out that Robin was one of the girlfriends in question.
Eric True Blood is an Anti-Shipper for Sookie/Alcide (and probably other Sookie shippings?), given that in the fifth season he hypnotizes Alcide into feeling disgusted by Sookie's touch.
On iCarly there were several occasions where one of the main characters ruined a relationship for the others:
In "iSaved Your Life", Sam puts the idea into Freddie's head that Carly is only into him because of hero worship after Freddie saved Carly's life.
In "iLove You", Carly torpedoes Spencer's girlfriend of the week relationship with a "The Reason You Suck" Speech, which simultaneously causes Sam & Freddie to rethink their own relationship as they overhear the speech and realise it applies to them.
Amy, Bernadette and Penny either want Leonard/Penny to happen or think Priya is out of line for trying to change Leonard and demanding he stop being friends with Penny.
Sheldon doesn't like her because she's mean to him and challenged the Roommate Agreement to help Leonard.
Friends: Phoebe is surprisingly anti Chandler/Monica, implying Monica can do better and predicting when they'll get divorced. This is very odd, as Chandler and Monica are extremely happy together and makes each other much better and more secure people. Yet Phoebe is the biggest Shipper on Deck for the dysfunctional Ross/Rachel, who aren't nearly as well-suited, and her weird perspective is never really explained.
Made especially weird because Phoebe's relationship with Ross could be described as Vitriolic Best Buds as the show went on, perhaps even moreso than her relationship with Chandler.
House of Anubis had many, at least one for every couple on the show. Most infamous, though, would probably be Joy, who made no secret of her love for Fabian or her dislike for him and Nina dating.
Charming's Jerkass father will take any chance in either world to separate the two, for a variety of reasons but mostly stemming from a vindictive streak after Charming refused to make a marriage alliance because he was in love with Snow.
While cursed, pretty much everyone is against Mary Margaret and David pursuing their inexplicable feelings.
If Regina trying to prevent Charming and Snow from being together isn't enough, she rips out the Huntsman's heart as punishment for him sparing Snow; forces the Huntsman, as Graham, to become her lover in Storybrooke (which essentially amounts to rape); and murders Graham out of spite and jealousy when Graham develops feelings for Emma. Regina generally seems to be the anti-ship to every canon ship in Storybrooke, breaking apart families, murdering, and harming others to make sure no one gets a Happily Ever After. Even when shown her soulmate, Robin Hood, Regina sabotages her own chance at a Happily Ever After, because she is afraid of falling in love again.
The Blue Fairy deliberately prevents Nova and Dreamy, later known as Grumpy, from eloping, because "dwarves don't fall in love", and she thinks she is protecting Nova for her own good. Hell, everyone (except Belle) seems against Dreamy and Nova's love for one another at first. However, after the Dark Curse is lifted from Storybrooke, Dreamy (now known as Leroy) and Nova are reunited, resuming their previous relationship.
On Graceland Paige and Johnny are pressuring Mike to break up with Abby. Their reasons are mostly pragmatic since Mike, Paige and Johnny are undercover federal agents and Abby is a civilian who does not know what they do for a living. Abby could inadvertently do something that would break Mike's cover and get him and the others killed. In turn, Mike always lying to Abby about his life and job is no basis for a real relationship and they want to save Mike from the heartbreak.
In Game of Thrones Catelyn Stark displays visible disapproval of her son Robb's starting relationship with field nurse Talisa Maegyr due to his existing betrothal. And hell, she's right...(In the books it's already too late.)
In the Firefly episode "War Stories" it's revealed that Mal used to feel like this about Wash and Zoe, since relationships among crew members complicate things.
From the pilot on for quite some time, Michael can't understand why Justin is different from any of Brian's other tricks, partly because he knows Brian so well and partly out of his own unrequited and long-unresolved feelings for Brian.
Amusingly, he shifts eventually sides and is vehemently against Justin and Ethan, something he would've welcomed not too long before.
Brian tends to sabotage Michael's relationships, whether out of a desire to keep himself as the most important person in Mike's life or just out of his utter disdain for monogamy.
Takes on a more serious tone when several characters (including his own mother) don't want Michael seeing Ben, who is HIV-positive.
Morgana on Merlin tries a lot of ways to break up Arthur and Guinevere, to make room for her to take the throne of Camelot.
A darker version on Kings with Jack and Joseph, as Jack's parents vehemently disapprove of his sexuality and certainly are not okay with him having an actual relationship with a man.
Eponine of Les Misérables is this towards Marius and Cosette, due to her own unrequited love for Marius.
Anita from West Side Story tries to sink Tony/Maria because she believes Tony is a dangerous, murderous boy who will break Maria's heart.
Art of Fighting: Takuma is aware of his daughter, Yuri, and his student, Robert Garicia, have feelings for each other. Her older brother, Ryo, is also aware of this... and they're both determined that it isn't gonna happen. For reasons unknown, they're convinced Robert will only take Yuri for granted, even though he helped rescue her in the original game and has been trying to get with her for the last several years. Likewise, Yuri is an adult and capable of defending herself. The issue was seemingly resolved, during Ryo and Robert's endings in Art of Fighting 3, where Ryo finally relented and told Yuri to go after him. But in the King of Fighters series, their relationship is downplayed and has remained tentative at best, due to Takuma's interference.
Girl Genius: Gilgamesh Wulfenbach and Tarvek Sturmvoraus are this to each other, since they both love Agatha), considering their backgrounds it gets put on hold at least until they can leave the battlefield where they were at the moment. Also they are inside a deadly architecture construct with a sadistic predisposition.
Larisa is this for a while in Sandra and Woo between Sandra and Cloud, until she realises she values her friendship with Sandra more than she wants Cloud as her boyfriend.
The Spectacular Spider-Man: Sally Arvil hated that popular, cheerleader Liz was in a relationship with geeky, science nerd Peter because it upset the social balance of Midtown High. Her reaction to their break up was to declare "finally", echoing Mary Jane's reaction to Gwen and Peter getting together.
Phineas and Ferb: Suzie always does everything in her power to break up her brother Jeremy's relationship with Candace.