It is my opinion that over-fixating on the terminology "primary" and "secondary" puts the actual point of the discussion on a back burner.
At least for me the issue is the view that with encumbrance comes greater value. If someone has kids with one person, that person is by default of higher value than someone they don't have kids with. If someone shares a mortgage with one person they are somehow "closer" than someone who they don't share a mortgage with. If someone has been with one person for 10 years the relationship takes higher priority than the relationship that has been around for 1 year.
Relationships are all different, they all have their good points and limitations. My relationship with my friends are highly valuable to me, as are my relationships with my lovers, for example. But if one of my close people gets sick, is hurt, calls me in need of help, you can bet that I'm going to ditch whoever I am with (friend or lover) to go to their aid. You bet your sweet ass. Their "rank" is irrelevant. I love my people, my friends and lovers, and I treat them with dignity. When they need me I go to them, when they are in trouble I fight for them, and I don't feel the need to place one over the other. Doing so devalues the relationships.
I wouldn't have you guys redefine your terminology (which I've had to Google not shy of a dozen freaking times btw), I only hoped to bring up the question, to shake the bees nest and get people considering. That's really all we can ask of each other, right? To try and keep growing.