If society grants status to pairs of people, then I agree that any pair of people (as Z notes, speaking of adults) should logically be able to grab the same status. Does it necessarily follow, though, that a larger group than two should get the same status? It is not clear to me that it does (nor is it clear that it doesn't).
I believe that if the reasons involved are the same...
Let me personalize this a bit.
In our household, there is me, my wife, and our roommate (To call her a 'lover' or 'mistress' or something would be misleading at the moment). There are the three children between myself and my wife; two children from our roommate's former relationship (wherein she WAS the mistress); and a shared child between myself and our roommate, conceived originally as a child for her and her (now estranged) husband, who was impotent (Since his leaving, I have adopted, renamed, and claimed full responsibility for our child).
My wife and our roommate both work. My wife is assistant manager at a clothing retail, while our roommate is an assistant manager for the airport's food concessions vendor at CVG. Both earn decent pay, both have decent benefits.
Among the six children, one has high-functioning autism, one has sensory integration disorder, two suffer from a number of physical medical problems, one quite probably will be a midget, and one has a number of psychological problems. The oldest are eight, the youngest, almost three.
Since my wife and our roommate work odd hours, I'm the primary caretaker for all six kids. I do all the cooking, cleaning, child care, potty training, preschool-running, home-schooling our SID child, taking care of the numerous weekly medical appointments and therapies, etc. Day care is simply not an option, whatsoever. And my earning potential, based on my military service, wouldn't even cover the basic day-care needs of the family, so my financial contribution to the family is as the day-care and household manager.
That being said...
Because I am the primary caretaker of the kids - including my roommate's two sons by her prior relationship - I am the one who has to manage most of their appointments. Children's Hospital Medical Center is a wonderful place, but they have a strict policy that the parent or legal guardian must be present. Thanks to their father, I cannot get recognized as 'legal guardian', as the courts have decided that I would need HIS approval to become their guardian.
I've also fallen to a grey area on medical coverage, in part, because my wife's company is stupid, and in part because my relationship to our roommate is difficult to understand. At the moment, I am covered under our roommate's medical, on the basis that I am father to one of her kids and chief caregiver of all of her kids; but it's as a 'significant other'.
I think in our case, a legally recognizable social contract would be ideal.
I don't think our family is typical, though.