Opposites Attract Revenge

A type of Love Triangle—heavy on the triangle, light on the love. When a girlfriend leaves her boyfriend, and ends up in a relationship with the boyfriend's ideological rival, resulting in a vendetta between the two men.

It works with the genders switched around, too.

Compare Murder the Hypotenuse. See also Cock Fight.

Anime and Manga

 * Spike and Vicious from Cowboy Bebop had one of these over Julia, resulting in the disintegration of their friendship and a lifetime of hatred toward each other.
 * Turkey and Greece from Axis Powers Hetalia were already on bad terms long beforehand, but them both liking Japan does not help one bit. According to France in the Christmas 2007 strips, it's bad enough that Japan spends most of his time with the two trying (vainly) to get them to be at least civil to each other.
 * Brutal example in the School Days anime. Makoto starts out interested in Kotonoha, gets help from Sekai, and later falls for her, then goes back to Kotonoha at the end after he suspects everyone ditched him due to his lecherous ways by sleeping around with all the girls in the series, and Sekai's declaration that she's pregnant.

Film

 * The Prestige.
 * Though the vendetta was pretty firmly in place before this came about.
 * Played brilliantly in Spider Man where Mary-Jane is constantly the center of the tug-of-war between Spider-Man and the Goblin, Jr. AND their alter-egos Peter Parker and Harry Osborn.

Literature

 * Harry Potter:
 * Also completely played straight since
 * Though, notably, they didn't start dating until their seventh year, when they were Head Boy and Girl and James had clearly matured quite a bit.
 * Alexandre Dumas' The Count of Monte Cristo. This is particularly pronounced in the latest Film of the Book, in which Mercedes is one of the key reasons behind.

Live-Action TV

 * Greek. Cappie's carelessness and jealousy leads him to beat up his best friend and roommate Evan for taking Casey to a formal when he failed to show. This leads to the pairing of Casey and Evan at the beginning of the series and the bitter rivalry between Evan and Cappie's respective fraternities ever since.
 * Smallville: Lex and Lana are an item for a time, resulting in upped tensions between Clark and Lex.
 * There is, of course, a Buffy the Vampire Slayer example. Tension over Drusilla's affections is the main source of the feud between Angel and Spike for much of the second season, even leading to Spike helping Buffy at the end of the season in the hope he'll get Drusilla back.
 * Ironically, Spike himself ends up romantically entangled with Angel's love interest, Buffy, in later seasons. These guys have VERY similar tastes in women.

Videogames

 * In the Phoenix Wright series, one of the cases involves a couple of dueling celebrities (one which is a Jerkass and the other which is a Complete Monster) and when the current fiancé (the Jerkass) found out that he had essentially gotten his rival's (the Complete Monster's) leftovers, he immediately called off their engagement. The Complete Monster had told him this just because he knew it would make his rival angry and this only escalated the rivalry between the two of them. The woman in the meantime? Tired of being The Chew Toy, she kills herself.

Webcomics

 * Missi and Emily in Misfile to the point where Ash tell both of them to bug off until they can be decent to each other.

Web Original

 * Dr. Horrible's Sing-Along Blog: The opposites in this case are the Villain Protagonist Dr. Horrible and the Hero Antagonist Captain Hammer; the Love Interest is Wide-Eyed Idealist Penny, and Dr. Horrible attempts to Murder the Hypotenuse to claim his love (while simultaneously proving he's a real villain.)

Western Animation

 * Happens in Total Drama World Tour with Duncan, Courtney and Gwen.
 * LaBarbara Conrad from Futurama has been married to both Hermes Conrad and his limboing opponent, Barbados Slim.