Ok! I think I found a piece of writing that succinctly and pretty fully covers my thoughts on the question of the utility of the idea of secondary relationships... it focuses a lot on how to handle secondary relationships so that they're healthy and validates that they're real, important, not disposable... and yet still different from primary relationships.
I find that way more useful than just trying to throw out definitions entirely and say that all labels are bad. We think in words, so having definitions helps us talk and work through things. I also find it way more useful than saying that all relationships should be equal. In my experience that just isn't the case for most people, even if it might be a long-term goal. Preexisting commitments and relationships that involve significant life-entanglement (marriage, house, kids) end up taking precedence, so whether you call them primary or not, they're different from newer relationships (whether you call those secondary or not!).
I want to feel proud of my relationship, proud of the definition that seems to fit it best, and proud of the name of this blog, even if I end up picking a new name eventually. This essay helps me do that.