When Across the Universe (2007) begins, Jude is happily dating Molly and Lucy is happily dating Daniel. Then Jude and Lucy find each other, and a plot ensues. Also, Sadie and Jojo, much to Prudence's chagrin.
Marty McFly and Jennifer Parker, as well as Emmett Brown and Clara Clayton, in Back to the Future. The former is already established as such since the first movie, further reinforced by the end of the third when Marty manages to prevent an accident that would have left him disabled in the future portrayed in the second movie (2015), thus leading to a more prosperous relationship in the new future. The latter is born as a result of a lucky change in history, as Clara was fated to die before Marty and Emmett saved her life in September 1885; they're revealed to have two children (Jules and Verne, collectively named after the science fiction writer) by the end of the third movie.
Final Destination 4: Nick and Lori are the only lead couple in the series who are in a stable relationship throughout the entirety of their film. They get a Sickeningly Sweethearts moment in their first scene, live together and make decisions together, and each seems to care more about the other's survival than their own.
The official couples in Harry Potter are, surprising absolutely no one, the same as in the novels, at least until the last film makes Neville/Luna an official couple.
Tony Stark and Pepper Potts are the Official Couple for the latter half of Phase 1 and all of Phase 2 of the Marvel Cinematic Universe. They get their Relationship Upgrade in Iron Man 2 before appearing as the Official Couple in both The Avengers and Iron Man 3. Unfortunately, at the beginning of Phase 3 with Captain America: Civil War, Tony and Pepper are separated partly due to Tony deciding not to retire from being Iron Man.Spider-Man: Homecoming and Avengers: Infinity War show that the two of them are back together and engaged. In Avengers: Endgame, they marry and have a daughter Morgan.
Thor and Jane Foster become an Official Couple in the MCU towards the end of Phase 2, after Thor elects to remain on earth and be with Jane at the end of Thor: The Dark World. Thor: Ragnarok reveals that they're not together anymore and by the time they meet again and reconcile in Thor: Love and Thunder, Jane succumbs and dies to cancer, leaving Thor single once again.
At the end of Avengers: Endgame, Steve Rogers goes back in time to be with Peggy Carter.
Peter Parker and Michelle "MJ" Jones are dating by the end of Spider-Man: Far From Home. However come Spider-Man: No Way Home, this ends up being undone due to Strange having erased everyone's identity of Peter in order to save Multiverse. When Peter meets her, he ultimately decides to leave her and Ned in the dark to keep them safe from other enemies that might exploit Peter's identity.
In Our Miss Brooks, the cinematic finale to the series of the same name, official couple Miss Brooks and Mr. Boynton finally get married.
Will Turner and Elizabeth Swann in the Pirates of the Caribbean films, complete with the most awesome wedding imaginable in the third movie.
Neo and Trinity in The Matrix are together throughout the original trilogy, and The Matrix Resurrections more-or-less explicitly confirms them as soul mates who will always find their way back to each other.
Sam and Quorra from TRON: Legacy. And in the first film, Tron and Yori - and their User counterparts Alan and Lora. The deleted scenes and novelization play it up.
X-Men: While not explicitly mentioned in the movie, Jean Grey and Scott Summers are described as each other's fiancé(e) in promotional materials.
X2: X-Men United: Rogue and Bobby Drake are in the early stages of a romantic relationship, and their challenge is dealing with her mutant ability, as she can kill someone with a prolonged touch.
X-Men: Days of Future Past: Iceman and Kitty Pryde in The Rogue Cut. In the Alternate Timeline, that seems to have been erased because Bobby is with Rogue again, and Kitty is presumably with Colossus.
Deadpool (2016): Wade Wilson and Vanessa Carlysle are engaged.
