A long time ago, I read something macabre that stuck with me: a new mother held her new baby and realised that although her child's father was the love of her life, she would gladly stand on his head to keep her baby from drowning. I love my husband, but yes. I can relate.
We fit the model of primary partners (shared finances, future plans, lifelong commitment etc) but for both of us, she comes first. Her needs are greater, her demands more vociferous and urgent, and our responsibility to her trumps everything else. It isn't that I love her more than him - I don't think I could assess which of them I loved more, when loving one of them just feeds straight back into loving the other.
I know that some polyamorous people are uncomfortable with the term 'secondary' and the concept of hierarchy in relationships. They feel that labelling their relationship as secondary to another makes them feel less important and maybe even disposable. I understand why someone might feel this way. I just hope these people never date anyone with young children.
We can't can't give everyone equal time, energy or affection. Hierarchy is present in nearly all human relationship, whether we give it labels or not. You know which of your friends are your best friends, and which are just casual acquaintances, even if you don't say so. There is a hierarchy implied by family relationships as well, so much so that when this hierarchy diverges from the norm, we point it out by saying something like "my grandmother was like a mother to me" or "she's my favourite aunt". Even monogamous people do it to some extent, by using terms to make the significance of their relationship clear, whether that is "just friends", "dating", "boyfriend/girlfriend", "partner" or "husband/wife".
It's understandable, however, that people are uncomfortable with this hierarchy being overt in their romantic relationships. Not only do we grow up expecting just one 'significant other' but we don't have a model for having more than one when we do. In our other relationships, a hierarchy is so ubiquitous that we barely notice it's there, but in love, its existence glares at us.
The fact that I prioritise my child over my husband, or my husband over my boyfriend, doesn't make either of them unimportant or disposable to me. My daughter needs me just to survive, and my husband's life is so intertwined with mine that all of my decisions affect him. What makes me prioritise others over my boyfriend (and therefore makes our relationship a secondary one) isn't a lack of affection; it's just that as much as I love him, his life is conducted largely separately from mine. My secondary status in his life (and his in mine) isn't a restrictive cage we've imposed on ourselves, it's just a fact. "Secondary" still marks our relationship as one of the most valued and important in both of our lives.
But I think it's also important to remember that some of these things can change. Previously only children might have to share their parents' attention with an even needier newborn. Long distance relationships can relocate. "Just good friends" can become lovers, and more. Secondary partners might stay that way because of circumstance or design, or they might develop into co-primary relationships.
And even my daughter's dominance over my life will change. My husband and I have made a commitment to stay together for a lifetime, but my daughter will rely on us less and less, and one day she'll grow up and leave home. Having been woken up by her kisses, I'm conflicted about that, but I know I'll let her go. The shape and structure of our relationships isn't always within our control.