I actually find the degree of negativity associated with the term secondary rather surprising. I guess a lot of people attach greater/different meaning to it than I do, and assume a hierarchy in all aspects, which I never mean to imply.
I call myself a secondary wrt my boyfriend, and him that wrt to me, because it seems a reasonable description of how much we are able to offer each other. In terms of time, family support, shared experiences, we simply have less than with our respective spouses. That does not mean we love each other less, or that we would say the other is less important to us, but marriage and family are in fact constraints. We both recognize them and tend to accept them, and while we both sometimes want more, I don't think we tend to think that the other is neglecting or not sufficiently valuing our relationship.
In fact its not a term we use in speaking to each other either; its just a (mostly) convenient shorthand on sites like this. Except that since the term doesn't bother me, I tend to forget when using it that it may trigger a lot of bad associations in others. For me it has no association of an imbalance or abuse of power.