I am in a relationship like this. HE is my primary, we openly discussed it and I said that is what I want his role in my life to be. I have no other relationships, actually this is my first poly relationship. However I am HIS secondary, though we have never actually voiced that in those exact words, as he is married. This fits fine for us, that is not to say it would work for everyone.
We don't use this hierarchy label to say "well we are the most important couple" or that we hold all the power in the relationships, it is merely a way for me personally to process the position my partner has in my life, our way of saying that our relationship is a serious one.
I have no issues accepting I will never be his primary, sure I have had to work through my own emotions about that, like what if I ever wanted more, there's nowhere else to take our relationship to, if you understand what I mean. But at the end of the day we personally don't need there to be anything legal stating we are serious, we know we are... not sure i am explaining that very well!