Pre-Crisis the Earth-Two versions of Superman (Kal-L) and Lois Lane were happily married for decades, and after Crisis on Infinite Earths they continued to be this while living inside a pocket dimension along with Alexander Luthor Jr. and Superboy-Prime. In fact, the whole reason Kal-L tried to help Alex and Prime bring back Earth-Two at the expense of the Post-Crisis DCU in Infinite Crisis was because he was led to believe doing so would save his Lois from dying. She dies anyway, but before she does she tells Kal-L how happy he made her and how grateful she was for these extra years. And at the end, after Kal-L succumbs to his injuries from a psychotic Prime, it's implied he's now reunited with Lois in the afterlife.
Post-Crisis Superman and Lois Lane starting from 1996. Initially, their marriage was undone when Flashpoint rebooted the DC Universe and erased them from continuity. Convergence brought these versions of Lois and Clark back by revealing they were trapped in one of the many cities Brainiac saved from erasure. When Convergence ended, the Kents and their new son are now living in the current DCU and have been for nine years, as seen in Superman: Lois and Clark and Superman (Rebirth).
Both sets of Superman's parents. His birth parents - Jor-El and Lara - were very close. In some continuities, Lara refused to leave Krypton because she would have to leave her husband behind. His foster parents Jonathan and Martha Kent are a very loving, very close-knit couple who have been married for decades.
Kara's birth parents - Zor-El and Alura - love each other dearly. In the Post-Crisis universe, Alura was completely shattered when her husband died.
Her foster parents (Fred and Edna Danvers in the Pre-Crisis universe; Jeremiah and Eliza Danvers in the Post-Flashpoint 'verse) are also very close-knit. Jeremiah and Eliza have been married for years and they think nothing of flirting to their boss's face while they are working.
In the Many Happy Returns storyline, New Earth Linda Danvers - a Supergirl who was human and unrelated to Clark Kent - spent five years married to Earth-One Superman after switching places with his cousin Kara.
Wonder Woman: On Earth-TwoDiana and Steve Trevor had a loving marriage not only for a human lifetime but beyond after being brought to Mount Olympus. Together they had a daughter Lyta/the Fury who continued the family's heroic traditions. They were also happily married on Earth-One, but Crisis on Infinite Earths happened shortly after their marriage.
Captain Atom: Nathaniel Adam was happily married to Angela Adam until the experiment that either blasted him eighteen years into the future and left everyone thinking he had died, or until he died for eighteen years and then came back to life (what really happened is open to interpretation). She had remarried a man he hated and then died of cancer in the meantime, and his learning to reconcile with her memory is a major part of his character arc over the course of the series.
There seems to be a tradition of this in the Flash legacy; see Wally West and Linda Park, and Jay and Joan Garrick, who have possibly the longest-standing marriage in the DCU.
In Rebirth, Barry and Iris began dating again, but the influence of Negative Speed Force powers and Barry keeping secrets from her has led to tension. Though once Barry loses the negative speed force powers and starts being more open things get better for them.
The Hawkmen and their partners (both at work and at play, as it were) used to have a similar happy relationship. Pre-Crisis, Carter and Shiera Hall of Earth-2 were married for about the same length of time as the Garricks, and used to go off on archaeological expeditions together in between JSA meetings. Likewise on Earth-1, Katar and Shayera Hol were married right from their creation. Post-Crisis (and post-John Ostrander and Hawkworld), Hawk continuity is just a mess, but Carter and the new Hawkgirl still get together fairly often. In fact the very first Hawkman and Hawkgirl's corpses are the central power battery of the Star Sapphire Corps (who are fueled by The Power of Love)
In The Authority, Apollo and Midnighter. Interesting in that they're both gay superheroes who are happily married. Frequently played with, too, as they still have squabbles, and the kid-equals-stability thing was utterly subverted in the Revolutions arc (though that was mostly down to Manipulative Bastard Arch-Villain Bendix). However, they've been together for a long, long time, and no matter what crap gets thrown at them, their relationship is consistently a very loving one.
This is such a major point in the Animal Man comics that it affects his costuming. He adds a denim jacket over his regular skintight costume so he can carry around his keys and notes from his wife. He's the only superhero with "bring home milk" on his crimefighting agenda.
JSA Classified: The reformed meta-human thief Johnny Mimic has been in a happy loving relationship with his wife Linda for decades. Once the government discovered his location they used that relationship against him in order to force him to use his powers for S.H.A.D.E. by threatening his wife.
The Legion of Super-Heroes: Lightning Lad and Saturn Girl, who were long-standing sweethearts since the Silver Age and the second couple to be married. When Saturn Girl became pregnant with twin boys, she was the one who decided to go back on active duty while Lightning Lad chose to stay at home to look after the boys as a House Husband. Their status as happily married was briefly undone during the tenure of Geoff Johns as writer for what was considered the original Legion, with Saturn Girl trying to keep her husband under control while he kept snapping at her for never taking his side. When Paul Levitz came back as Legion writer, Garth's anger was toned down and the Ranzzs have gone back to being a supportive couple, this time with both of them retiring from active duty to raise their boys on Garth's home planet Winath.
Also from the Legion are Bouncing Boy and Triplicate Girl, who were the first Legion couple to get married. In every continuity where they're married, they've never been shown as anything but absolutely crazy for each other.
Scott Free and Big Barda prove that a beautiful relationship can come from the darkest of places. They first met on their horrendous home world of Apokolips. They quickly fell in love but he escaped Apokolips before she could. She eventually followed him to Earth where they got married. This happy marriage has followed them since their 1971 introduction to this day in both comics and adaptations. Barda is based on their creator Jack Kirby’s wife Roz to whom he was happily married for over fifty years before his death in 1994.
Black Order: Debuting in Infinity, villain married couple Corvus Glaive and Proxima Midnight have an openly loving and affectionate marriage. They are minions of Thanos, literally worship death, and are bloodthirsty and cruel and love slaughter...but this doesn't stop them calling each other by pet names like "my love" or "dear midnight" in the middle of battle. Proxima even contradicts their whole 'love-of-death' belief by shutting down mid-battle because Corvus was vaporized.
Dark Reign: In Dark Reign: Fantastic Four #2, Mr. Fantastic views one alternate universe where Captain America and a female version of Iron Man are this. Their happy relationship is the main reason that Earth-3490 is a utopia.
Fantastic Four: Reed Richards and Sue Storm-Richards tied the knot in Fantastic Four Annual #3, back in 1965, and they've been together ever since despite bumps. Namor (among others) has had an on-again-off-again Stalker with a Crush thing on Sue for a long time, and it hasn't always been completely one-sided.
The Incredible Hulk: In Planet Hulk, The Hulk was this at the end, with his new queen, Caiera. It’s notable because not only was she powerful enough to be somewhat of a challenge for Hulk, but she also loved both Hulk and Bruce Banner.
Runaways: Karolina and Xavin are also not married, as their wedding was interrupted by the planet they were on exploding, but they were Happily Engaged up until Xavin was Put on a Bus. The six couples that make up the Pride also count - while they're all murdering, thieving supervillains, their marriages and family lives are quite healthy (except for the way the Steins treat Chase, but even then, they love him and are willing to die for him).
Peter Parker and Mary Jane Watson until the One More Day comic. They're also married in the newspaper strips, and have spent thirty years together there. This is parodied in Ultimate Spider-Man, in which Mary Jane and Peter are so close that Liz Allen starts to refer to Mary Jane as "Mary Jane Watson-Parker" because "You two are totally married!" Peter even tells MJ at one point that when they become old enough he would marry her.
It's kind of telling now that they're back to being an Official Couple in The Amazing Spider-Man (2018) that most people assume it's a matter of when and not if they'll be married again. In fact, Issue 29 of the volume revealed that Peter almost re-proposed to her.. The finale of Spencer's run also reveals that Peter and MJ will have a daughter that will finally end Mephisto's reign of terror
Likewise, Uncle Ben and Aunt May had a long, happy marriage before taking in Peter. Their backstory reveals that before getting married they actually had a Will They or Won't They? relationship which May compares to Peter and MJ's.
Cyclops and Jean Grey until Jean's death in New X-Men. She got better several decades later, and currently enjoys married life with Scott again on Krakoa, though it is implied she occasionally flirts with Logan on the side with Scott's knowledge (and permission)
Mystique and Destiny, Rogue's adoptive parents, had an amazingly long loving relationship that began when the two women met in the 1890s and only ended when Destiny gave her life so that Mystique could live. Although it is known that through some unexplained circumstances, they were separated for unspecified times, there was never seen a cross word between them. Pretty good showing, especially considering they're both villains.
Rogue and Gambit got married in 2018 and are still together as of the Krakoan Age, with their interactions with each other showcase very effectively just how long they have been together, explicitly and otherwise.
Young Avengers: Teddy Altman (Hulkling) and Billy Kaplan (Wiccan) are well on their way to filling this trope. They're already the longest-lasting couple on the team, and they've been engaged since they were sixteen. Even a universe-ending parasite and mutual fears that Billy was accidentally manipulating Teddy with his reality warping powers ultimately weren't enough to break them apart, with their relationship stronger than ever after working things through.
Other comics
In PS238, Malphast's parents are quite happy together, although, given they're personifications of Chaos and Order, it's not entirely clear if they're actually married. But if they were humans, they'd probably be a model family.
Appears fairly frequently in Astro City, due to its overall optimistic nature.
From the First Family, there's Natalie Furst to Rex Zorus, the monster prince of an undersea kingdom. Rex defied his own mother to marry Natalie, and the entire extended family has remained cordial for over twenty years.
Zachary Johnson, the second Jack-In-The-Box, has been married to local television news anchor Tamra Dixon for decades. He gave up super-heroics after finding out she was pregnant, and have been nothing but supportive of each other.
Michael Hendrie (M.P.H.) is briefly mentioned as being happily married to a woman named Sally. Their relationship is not shown, but given how M.P.H. has been seen treating his girlfriends, it's hard to imagine them having any sort of drama.
Duncan Keller, a.k.a. Starfighter, is married to Illula, Seven-Fold Empress of Jarranatha. They have a son and daughter, have been together for nearly fifty years, and still shamelessly flirt with each other.
Thatcher Jerome, a high-ranking member of the Deacon's mob, is devoted to his wife Rachel, even into their senior years. Thatcher turned down a chance to give himself super powers because he didn't want to disrupt Rachel's contented life.
Archie's parents, Betty's parents, Jughead's parents... even Veronica's parents, despite being mbillionaires, aren't using that money as an excuse to become a stereotypically disconnected rich couple.
Reggie may be a jerk, but he can't blame it on a dysfunctional family.
The only exception was Midge crying because her parents were on the verge of divorce and even then she said they would stay together "with counseling".
Count Max and Ruby, the Vampire King and Queen of the Night from Scary Godmother, are head over for heels for each other. It's a case of Opposites Attract: Max prefers things old fashioned and likes adhering to traditional ways while Ruby is something of a modern woman who has no trouble adjusting to change. In one comic the two have a brief fight over traditional versus modern ways, and while separate, they see things about both lifestyles they don't find attractive at all. Max is followed by fan girls who want to obey his every command, which Max is reminded is something that was considered an old fashioned trait in women but which he thinks is disgusting. Ruby is accosted by several "modern" vampires who make rude comments and treat her disrespectfully. This also reminds Max and Ruby of why they love each other. Max thinks Ruby is vibrant and energetic and is attracted to her determination and ability to think for herself. Ruby loves Max's respectful demeanor and the way he both genuinely compliments her and treats her like a goddess.
Circles: Paulie and Douglas, a gay example played absolutely straight. After Douglas broke up with his girlfriend, she never told him she was pregnant and had their son until after she died and gave up custody. Douglas' son had no hard feelings and still liked his dad especially because he saved his mom from a loveless marriage.
ElfQuest has a lot of happily married couples, despite the elves' open sexuality. Among the elves there are Redlance and Nightfall, Strongbow and Moonshade, Woodlock and Rainsong, and Cutter and Leetah. You could also add Bearclaw and Joyleaf - they do have some pretty fierce quarrels, but always make up afterward. Also the trolls Picknose and Oddbit (really!) and the humans Nonna and Adar. In the event of romantic rivalries, forming threesomes is considered a perfectly acceptable, if relatively rare, solution.
What's more, in a sense Cutter and Skywise are "married" - yeah, they're both guys (though remember, in ElfQuestEveryone Is Bi) but in many respects they act like a married couple.
In Fables Beauty and Beast definitely qualify, despite the occasional bickerings (as Beauty points out, you can't expect a marriage to last for thousands of years without quarreling). Snow White and Bigby also qualify later.
Swedish comic book Bamse has the eponymous character happily married with four children.
Cartoonist Charles Addams invariably depicted marriage as a literal battlefield, with the notable exception of his reoccurring characters who became The Addams Family: in their own very unique way, proto-Gomez and Morticia are a loving and devoted couple.
The 20/20 oneshot of Jem and the Holograms (IDW) shows Jerrica and Rio have settled down in the 20 years after the events of the original series.
where Captain America and a female version of Iron Man are this. Their happy relationship is the main reason that Earth-3490 is a utopia.