I am in a situation where I have a boyfriend of 2 years, and 2 relationships where I consider myself secondary although we have never mentioned the terms "primary" or "secondary". I guess it's more my own feelings, since I came into the picture after they were already in relationships (one of 15 years, A, and one of 1.5 years, K), so I have the feeling I cannot ask for anything more than what I am getting, since their other partners have a bigger stake/right.
Additionally, A is married to his high school sweetheart and has 2 other relationships on the side, both that started before me, so it's hard not to feel like "just the third side-gf". And he stated clearly that not falling in love is one of the rules he and his wife have in place. So even if I am maybe developing some more feelings for him, I won't ever voice them, and live with the fact it's just transitory (which he specified, when he and his wife will have kids, they will most likely break up their other relationships).
With K it seems that the story is developing slightly differently, maybe because both he and his gf have much more experience in open relationships and know better what to expect/how to handle these situations. Plus he seemes keener on having two equal relationships with his gf and me. In this sense I consider myself secondary with the option of eventually reaching parity (once the relationship is more established, if we both want to).
I consider my boyfriend my primary partner again simply because he was here first. He also has no interest in pursuing other relationships at least for the time being (let's say that he has to live with the fact that I want an open relationship, it wasn't his idea. But he's coming on board...)
It helps that I live alone and have no desire to share living conditions, finances, etc with any of my partners.
I do feel sometimes I would like to be more expressive in my feelings for my secondary partners, but I restrain myself in the interest of their primary relationships.
I guess as nycidie says, it might stem from what we've been taught to expect from relationships, but overall I am quite happy with the status of all my relationships.