This was also my preference, at least until recently.
I actually tried this for several years with my previous relationship, after giving co-living a go (and failing at it) for the first two years.
On hindsight, this probably made it workable for longer, but that was not necessarily a good thing: we probably should've gone our separate ways sooner. Living apart together made more tolerable dynamics that were not conducive in the long run, even if the connection and good intentions were there… At least we tried.
The above example is not ground to dismiss such a model. I still think highly of it. Whatever works best between people. Cost-efficiency comes second.
However, it can sort of hide underlying issues (who doesn't have any?) that could be best worked on at an individual and interpersonal level.
In my mind, this "requires" at least two fairly functional, open-minded and independent adults, who do it both for preserving their own space and for helping their relationship find a "space" of its its own and grow. If it is only for "me" reasons, then it can create a kind of imbalance.
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@Lemur, the Iroquois example made me think of Bill Plotkin. Maybe it was not an introverted excuse for alone time, and they were just healthy adolescent members of an ecocentric society, feeling compelled to pursue vision quests during the Cocoon stage of human development? Ha!
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GEORGE BRASSENS
The best example I know is that of the famous non-conformist French singer/song-writer George Brassens and his lifelong (like for more than 30 years) muse and fiancée, Joha Heiman, or Püppchen, little doll, as he affectionately called her, They remained childless, never married, and always lived together under separate roofs.
He wrote for her one of the best love songs I know, "La non-demande en mariage" (The non-marriage proposal).
English translations found online being not satisfactory, here is a home-cooked one:
George Brassens - La non-demande en mariage:
https://www.youtube.com/watch?v=bCxWiAFmjgY
THE NON-MARRIAGE PROPOSAL
My dear, by grace, let's not put
under Cupid's throat
His own arrow
So many lovers who have tried it
Have paid with their happiness
For this sacrilege…
I have the honor not to ask for your hand
Let's not put our names on a parchment
Let's give a free pass to the bird
We'll both be prisoners
On parole*
To hell with housewives
Who lead hearts by the handles
Of their pots
I have the honor not to ask for your hand
Let's not sign our names on a parchment
Venus often gets old
She's at loss in front
Of the stove
For nothing in the world do I want
To play the daisy game
Over the stewpot
Melusine* loses her charms
If one reveals too much of
Her secrets
The ink of love letters fades
Quickly when folded between
Cookbook pages
It may seem harmless
To preserve
In a jar
The lovely forbidden apple
But once cooked, it has lost
Its natural taste
I have no need of a maid
And I free you from any household
Chores
May I always think of you
As my eternal fiancée
The lady of my heart
I have the honor not to ask for your hand
Let's not sign our names on a parchment
*Here, "parole" also means "oral promise". Interestingly, in English, parol without the "e" = a verbal agreement, or a written one that is not official
* Melusine -
https://en.wikipedia.org/wiki/Melusine. Figure of European folklore, a female spirit of fresh water in a holy well or river. In one of the French folktale versions, she married Guy de Lusignan, Count of Poitou, under condition that he should never attempt to intrude upon her privacy.