Where did ponies - and other species - come up with a shared idea of being married? And why is it generally presented the same at the core across all the in-universe cultures? Isolation between sapient races spans various ages, and in all pieces of writing that I have spanning before the Collapse (and even before that) one of the earliest societal keystones described are unions and spouses and the like. It's only briefly mentioned and attributed to them, but concepts of weddings and marriage would have their foundations in divine and immortal practices. This would make the tradition to formalize and celebrate mated pairs so distinctly and specially a (mostly) Alicorn series of traditions. Mortals would have just been inspired and awed by the practice to the point that their cultures would have made their own diverse equivalents in their own civilizations.
The core ideas of the practice would still be (mostly) universal, no matter who adopted the bond. Though, the exact significance would have a variance to different cultures. In the age of the Three Tribes, the unicorns would have seen it as a legal contract to swap property. For Alicorns, it would always be the greatest sign of devotion among lovers, and it wouldn't be taken lightly (why Alicorns tend to settle down with one other for eons and eons) because of how they founded things.
A simple (but incomplete) list of species transformations performed when one steps through Lumina's Mirror (the mirror portal) and heads to Sunset Shimmer's adopted world:
Ponies ---> Ordinary Humans
Alicorns ---> Ageless Mages (of varying strength, but sorcerers all the same)
Draconequui ---> FUCKING SHAPESHIFTERS
Dragons ---> Dogs
Demons ---> Cats
Griffons ---> Non-Vegan Humans (What did you expect?)
Phoenixes ---> Phoenixes
Breezies ---> Fairies
Kirin/Qilin ---> Werewolves? Something confusing? I honestly never finalized what they might be.
Seaponies ---> Mermaids
Sirens ---> Excessively Youthful Magic Humans
Buffalo ---> Ordinary Humans
Windigo ---> (Most likely) wraiths, wendigo, some kind of primal spirit
Undead ---> Still Undead
Reapers ---> Grim Reaper-type entities
If this doesn't start to shed some light on why I never expanded on my Equestria Girls stories and that side of the mirror as I did with the ponies, I'm not sure what else I can do to except spell it out: any series (nowhere near what I would have done in terms of complexity for the main stuff of the 'verse but certainly some novellas) would have been very elaborate in terms of the amount of planning that would have gone into these. They canon would have diverged so sharply from the first three movies into something resembling urban fantasy, likely with a focus on the human counterparts of the princesses, Sombra, Discord, and Sunset Shimmer.
The central idea would be Sunset having to go into her strange new world beyond high school and uncover a variety of mystery and danger, with some parallels to the other stories from the horse side of the mirror, driven by her not/being unable to go home again. This world's Celestia and Luna live and operate quite separately outside of the high school side of their lives, beyond just having different homes.
Celestia and Luna were apprentices to a wicked sorcerer (Starswirl) in days of old, with an Arthurian vibe to their past. Luna operates as more of a witch, and like her equine counterpart, she is the more powerful of the two sisters. Sombra acts as her beloved cat and familiar (thus their bond is platonic in this universe) and is general very goth-y outside of professional principal times. You can bet she lives in a big Victorian manor filled with all manner of eccentric things. She never fell to any kind of Nightmare Moon state, but often struggles with mental health, and has spent centuries as a wanderer on her own. Reuniting with her sister to keep a high school front around the portal and maintain a vacation-like ordinary life from their different travels is a break to her, and one that has allowed Luna to make peace after previous fights that can be expected from sisters prone to spats as any other.
Luna adores magic and adventure, and is often shy. The woman lives a fulfilling live alone as an aromantic asexual with aspirations to have as many experiences with different cultures and places as possible. She has an affinity for the stars and the faint desire to know other worlds and a whimsy about her that she doesn't show other people. She is 1000% a cat person. Her love of mysteries and strong sense of justice has led her to investigate historical mysteries and act as a vigilante. She has neither fond nor foul feelings for other humans, and keeps herself away from the brunt of society in order to keep her solitary lifestyle as she wishes.
Sombra and Celestia don't get along, but this dislike pales in comparison to their equine selves. Sombra distrusts Celestia and has no fondness for her and her ideals. Celestia isn't a cat person, but otherwise has little major grudge with the cat. She totally thinks he's a demon, though, completely unlike her graceful companion, Philomena.
Without the paths in her life that her Alicorn counterpart has, or the many events and traits, this Celestia enjoys a fairly quiet immortal life. She's simpler, a pale reflection of the complicated mare on the other side of the mirror, but she still has some of her traits: her composure, many of her likes, a fraction of her magic, some of her morals, and some of her regal airs. However, she is mellow and only puts her purpose in simple living in the many mundace unskilled (and sometimes odd) professions she works at. She is a bit too optimistic, but lacks the urge to meddle, protecting her from many sorrows as a result of her optimism.
She becomes willing to take on Sunset Shimmer as an apprentice in her post-high school years, knowing the younger woman has nowhere else to go while her friends advance in the wider world. Unlike Celestia and Luna, Sunset has not had the ability to ingrain herself in society as an unusual lurker, and remains without the means to live undetected as an anomaly/suddenly person from going through the mirror all those years ago. Without family, and with the knowledge that she likely would have to formally relive some of her she-demon crimes in a court AND be caught up in the turmoil in Equestria post-Enemy of Mine, Sunset tries to live under the roof of Principal Celestia, light mage and many other things. She's the self-appointed guardian of the mirror portal.
Principal Celestia lives in quaint suburbs, dipping in and out of the normalcy she enjoys and away from her sister's rural home. Living with her are her phoenix, apprentice, and spouse, the shapeshifter and taxidermist, Discord. They don't really fight crime. At least, not normal crime. She is deeply torn about whether she should let fairies stay in her garden or if she should fetch her broom. This Celestia is still deeply afraid of spiders, and even more afraid of the world's most neutrally aligned forces: the Reapers, Tirek, Scorpan, and their sister. (Neither of the brothers have ever fallen to villainy in this world.)
As the years pass, Sunset lives out the rest of her life as an ordinary human (though, one with a bit of Equestrian magic left) and helper to Principal Celestia, growing apart from any connections of the pony world she always longed for and still loved until her death. The rest of her life, from potential spouse(s) to the adventures she would have had in-between all remain unwritten, but I still have the notes, scraps, and other ideas that fueled this entry of Failed Footnotes. Though, I still toyed with a setting that was strictly more into lighter urban fantasy/magical girl type of stuff that resembled the post-Friendship Games direction/genre slightly more. All in all, any Equestria Girls material from me tends to echo my more fleshed out plot bunnies regarding the Principal Sisters as sorceresses.
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End #3