Primary is to central as secondary is to _____________?
I've always been opposed to the idea of hierarchy, but when you're living with partner(s) and have responsibilities around money and basic survival, it's necessarily more involved than a partner you don't share those things with. I'd really like to see labels built around those differences instead...
"Hi, new friend! This is my partner, Jack, and my partner, Jill, who is also my _______________ because we own a house together." (or raise kids together, etc.)
Home plate (methinks poly baseball analogies are fertile ground)