The Avengers: In All-New, All-Different Avengers, despite the Ship Teasing, nothing seems to be happening between Cap and Thor. After Cap found out she is Jane Foster, she essentially told him that they won't be able to have any relationship, as she's a cancer patient. And in Cap's own comic, he starts a relationship with Misty Knight.
In-universe, Kamala furiously attempts to torpedo a Fan Fiction featuring a love triangle involving herself, Spidey and Nova.
Hawkeye: An issue of Hawkeye (2012) seems to do this to all three of his most prominent romantic relationships: he cheats on Jessica who breaks up with him, finally signs divorce papers for his marriage with Bobbi, and Natasha is thought of as his work wife.
Every new creative team since Vaughn and Alphona's departure from the series has made sure to sink at least one ship during their run. Joss Whedon sank both Nico/Victor by having Nico willingly step aside to let Victor pursue Lillie McGurty, and put a good-sized hole in Nico/Karolina when Karolina tells Nico that Xavin "gives me something you can't."
Later, Terry Moore all but obliterated Karolina/Xavin by having Xavin take Karolina's place when she was accused of war crimes.
Kathryn Immonen apparently tried to sink Nico/Chase by having Chase get hit by a car after possibly running into Gert, but then the series was cancelled. Nico/Karolina took another broadside, after the Runaways appeared in Avengers Academy, which led to Karolina hooking up with Julie Power.
Secret Wars (2015): A fandom favorite in Marvel Comics was between Robbie Reyes, the All-New Ghost Rider, and Kamala Khan, the All-New Ms. Marvel, even though they never met. The one-shot Secret Love finally makes this both Ascended Fanon and this trope when the two team up and Kamala proceeds to declare them "Crossover Besties 4 Lyfe!", much to the relief of their canon love interests.
There was huge May/Brad shipping. At least until she discovered he hates mutants and people with superpowers. From this time she can't even stand five minutes near him. Tom DeFalco was playing Will They or Won't They? for years, and sunk it in one issue. And it worked!
Spider-Man: Marvel has attempted to break up Peter Parker and Mary Jane Watson many times even before they got married:
Originally Mary Jane was introduced as a girl that Aunt May believed would be good for Peter but whom Peter refuses to even meet, as he assumes this girl couldn't be cool if old stodgy Aunt May likes her. Later issues had MJ appear as The Faceless, but context details confirmed that she was incredibly beautiful based on the reactions of other characters. By the time Mary Jane makes her first appearance, and got a design that audiences loved at once, Lee-Romita felt they needed to go in a new direction and tried to make Gwen the Love Interest, but audiences gravitated to Mary Jane who Peter dated for a few issues. So Stan Lee decided to have Peter call her irresponsible and so on, and then had her Put on a Bus with a ridiculous hairstyle in later issues and so on. The romance of Peter and Gwen in the meantime was unpopular, and ultimately Gerry Conway killed off Gwen and made Peter and MJ a couple.
Marv Wolfman got tired of the Peter/MJ romance that had been going strong for more than fifty issues at that point and saw fit to break it up. He decided to have Peter propose to her and have Mary Jane reject him, which he believed would make readers unsympathetic to her. Of course readers felt that Peter proposed in a cheap way (leaving a ring in a cracker box) in Issue #182-183, and Wolfman himself provided Mary Jane a compelling Freudian Excuse (revealing that her parents were divorced) in Issue #191-192, after which she was Put on a Bus. How did that work out? Well exactly 100 issues later she said yes, and got married to him.
Roger Stern, a writer who liked Mary Jane as a character but didn't think she and Peter were compatible, reintroduced her as a friend in Issue #242 and set up a backstory for her about a broken home and an abandoned sister which hinted at guilt and regrets, as well as family loss. After he left, his basic outline (Mary Jane had known Peter was Spider-Man, her childhood had given her baggage and that her public persona was a mask) was repurposed by Tom Defalco in Issue #258-260, and this started a new period where they became best friends and confidants, with Mary Jane now proving to be a Foil for Peter and Spider-Man in having a double life, and this ultimately made them far more compatible than they would have been without this backstory. And in less than 50 issues they got married.
After they got married, writers came up with bizarre plans since now sinking the ship meant either divorcing them (which made Spider-Man look bad as a family friendly character and also aged him), widowing Peter (which meant making him a Failure Hero on such a level that no one could buy him being a quipster or dating again after losing the love-of-his-life, and also aging him). So they tried byzantine plots like The Clone Saga, where Peter 'retired' as Spider-Man so that his clone Ben Reilly could take over as the main character. After Ben's death and Peter's return, they tried killing MJ in a plane crash, then had her return but separate from Peter, but it didn't stick as the two patched up their relationship. Then the infamous One More Day storyline broke them up for well over a decade, albeit the manner in which it was done immortalized the pairing since EIC Quesada admitted that he couldn't find any real in-story way to end the relationship that made sense with the characters and backstory and felt it was a corporate decision to keep Spider-Man a Kid-Appeal Character.
In the Post-OMD era, they tried to sink the ship by simply refusing to let it sail. Much time was spent keeping MJ out of the books, having both her and Peter in different relationships, neither of which were compelling or charismatic enough to replace the earlier one. Instead attempts were made to Ship Tease them to bring readers in, only to Bait-and-Switch it with gimmicky storylines. Dan Slott's run had Peter's then girlfriend Carlie Cooper and Mary Jane in the wake of Superior Spider Man abandon Peter agreeing that the price was too high to be in a relationship with him. Mary Jane transferred over to the Iron Man books and has only sparingly interacted with Peter since, with their relationship routinely jumping from friendly (Chip Zdarsky) to tense (Slott) Depending on the Writer. After Slott left, The Amazing Spider-Man (2018) brought them back together.
Zeb Wells decides to basically nuke the ship in his run, The Amazing Spider-Man (2022), by starting with a Time Skip of six months, then showing Mary Jane raising kids with new character Paul. And it's eventually revealed that they're married.
The major problem with sinking the ship is that Mary Jane is Peter's major love interest in most adaptations and in alternate versions such as the Newspaper Strip, the Ultimate Spider-Man titles and she is also the mother of Spider-Girl, Marvel's longest running title for any female character. So popular are they that writer Dan Slott revived them in the alt reality family title The Amazing Spider-Man: Renew Your Vows which sold incredibly well and started a series. In other words, there are rarely any long gaps without some version of Peter and MJ either married or in a romance in some Spider-Man comic or the other.
The Cyclops/Jean/WolverineLove Triangle has had this happen to it dozens of times, usually due to Jean picking Scott over Logan or one of them getting killed. Though there's always a Shipper on Deck like Mr Sinister for Scott/Jean who attempts to bring them together via resurrection before continuity or writers rip them apart. Scott had done this way more times, sinking his relationships with Madeleine Pryor and Emma Frost due Evil Twin heel turns or being dead.
Wolverine/Mariko Yashida's relationship ends tragically. Not only does Mastermind throw a spanner in the works by using his mind manipulation powers to make Mariko call off the wedding, but Mariko is (much later) poisoned by rival crimelord Tsurayaba, and to avoid a painful death she asks Wolverine to end her suffering which he does. There were many Manly Tears.
Avengers vs. X-Men has two of these: Black Panther and Storm, Rogue and Magneto.
Storm's relationship with Black Panther ending was especially tragic as fans mostly liked it since it's inception. However thanks to massive damage and body count in Wakanda because of the Phoenix Force and the part the X-Men played in the slaughter, Black Panther annuls their marriage and tells Storm she isn't welcome in Wakanda. A fight ensures, and it ends very bitterly with some tear jerking flash backs to happier times.
A more bittersweet example is when Magneto proposes to Rogue after they save some civilians from a evil Phoenix Forced-influenced Cyclops, Rogue takes the ring but after spending the night and a rescue mission together Rogue gently turn down Magneto's proposal.
Subverted with Gambit and Rogue as similar to Magneto, Rogue leaving Gambit in Antarctica after sleeping with him appeared to be end of their relationship as the writers and good chunk of the readers assumed. While it did leave its mark, the relationship eventually recovered and both characters seem perfectly content to forget it ever happened.
In Ultimate X Men, the Nightcrawler/Colossus ship was brutally sunk when it was revealed in issue #67 that despite being persecuted for being a blue demonic mutant and otherwise being one of the purest nicest guys on the team, Nightcrawler is suddenly a homophobe with a creepy obsession with Dazzler.
Monet St. Croix & Sabretooth is a more subtle case. They have an overt Ship Tease during Uncanny X-Men (2016) and Weapon X (2017). Numerous characters, consisting of Psylocke, Callisto, Emplate, Deathstrike, Domino, Deadpool, and Omega Red, believing they have feelings for each other. The ship not only sinks, but is brutally shot down when Creed's inversion is undone the final issue of Weapon X. He's now sided with the Malekith during the War of the Realms and has plans to exact bloody revenge on the X-Men. So needless to say, anything he may or could have had with Monet is over. Sadly, Monet is in the dark on his reversion & was last seen wondering what became of him.
In the mainline Marvel Universe, Jim Shooter wrote a romantic sublot for Colossus in the middle of Secret Wars (1984) for the express purpose of finally putting an end to the Kitty/Piotr relationship, a longstanding point of complaint by Shooter (for good reason — his concern was not with the relationship itself, but with the fact that Pryde was at the time 14 and Rasputin was 19, and so was concerned with reasonable worries about how the relationship would be perceived by the readers.) The followup issue of Uncanny X-Men was Claremont taking the plotline and running with it. Despite this, Joss Whedon tried to bring the relationship back during his Astonishing X-Men run. At that point, Kitty was an adult so the age wasn't a problem anymore.
In X-Men Forever, Kitty starts to have romantic feelings for Gambit. Remy turns her down, pointing out that at the time this is going on, she is going through a massive amount of mental and physical trauma concerning her assimilation of Wolverine's claw and powers, and that she is not thinking straight.
In X-Men: Gold #29 and #30, it's seem like Colossus and Kitty are finally, finally getting hitched but when Piotr tries to put the ring on her◊... it doesn'twork out. Despite genuinely loving Colossus, Kitty can't form a commitment after such a turbulent history, thus making any reunion unlikely any time soon.. But for Gambit and Rogue, on the other hand, uncertainty and turmoil ironically is the basis for their relationship in the first place, so they decide not let the reception go to waste and get married with Kitty and Colossus' blessing.
◊... it doesn't work out. Despite genuinely loving Colossus, Kitty can't form a commitment after such a turbulent history, thus making any reunion unlikely any time soon.. But for Gambit and Rogue, on the other hand, uncertainty and turmoil ironically is the basis for their relationship in the first place, so they decide not let the reception go to waste and get married with Kitty and Colossus' blessing.
