Well the thing is, Ma and Pa Gorg started out as a married couple; it was integral to their characters from the beginning. But often on TV shows when a couple finally gets married, the story becomes less exciting because basically you've reached the climax the show has been reaching for all this time. Now what? Lol
The only married Muppets on Sesame Street are parents of the other kids. The Bear family, granted, is the only one seen regularly... but then of course there's Elmo's parents. Any other group of parents is only shown in the kid's books. And while there are some clearly adult Muppet characters, like Count vonCount, Oscar, and Guy Smiley and all of them, Ernie and Bert, no different from Cookie Monster, Grover, Herry, or Telly are ambiguously aged. The only characters with a set age (even though a lot of them are supposedly written to be a specific psychological age, though I still don't see it) are Elmo (Three and a Half... no one realizes how many Looney Tunes references they hide in there) and Big Bird (6, right?).
With Maria and Luis, they're human. They've aged since the show began. They were young adults when they joined the show, Maria was almost a couple with David in the early days, then they got married when the show producers thought it would be a nice way to talk about it, ditto when they had Gabi. And Gabi aged. Now, we know that puppeteers have died or left, and characters have been rebuilt, but other than that, they're frozen in time the way cartoon characters are. No greater example stands than Baby Natasha still being a baby.
When I did all that "grueling" research on YT for Oscar clips for my Father's Day in a Trash Can fic, I realized that Oscar's sense of Grouchiness actually forces him to live in a nice (relatively) neighborhood and do other things that, while he gripes about it, gets on other grouches' nerves. If Grundgetta shares this need to be different from other grouches, they might end up with a perfectly happy marriage, because to be different is precisely what Grouches do.
I don't know if it was just Carol or anyone else who came up with it, but he perfectly states the Grouch Paradox that being angry makes grouches happy, but being happy makes grouches mad, but then they're happy that they're mad. Something tells me that Jim and the rest weren't thinking how deep Oscar could be to begin with, and they just wanted a grump type of character... one that existed since, at the earliest, Grumpy the Dwarf. But then they came up with grouch culture, the paradox, their own special Grouch land... and that does add depth to not only a character, but a concept. Yet the concept is incompletely defined, so we make the rest up ourselves.
Maybe Oscar's the next level of Grouch... he hangs around with nice people because their happiness makes him more grouchy than hanging around with grouches... yet he does still eat grouch food.... that gets me to think, does a grouch constitution digest icky food perfectly because they've been eating that stuff forever, or does it give them very upset stomachs and intestinal pain which they like because they're grouches.
As you can see, grouch culture is a very interesting subject and it leaves so much for the fans to try and figure out... so I wouldn't be surprised if they have a happy marriage JUST because it makes them even more grouchy than a dysfunctional one.