I don't want to get into a long argument about descriptive versus prescriptive hierarchy, but I don't honestly think it's so terrible if everyone is ethical about it and up front about their wants and needs and boundaries.
I'm married. My husband and I chose to get married as poly people, as an affirmation that we plan to build our lives together. We have financial entanglements and a family together- so to us, it's simple reality that the majority of our emotional energy goes into our relationship.
When we date, anyone else is going to be a secondary partner since we just don't have the time and physical or emotional resources to build a relationship to the co-primary level. We are not looking to entangle lives with anyone else, nor am I looking for another partner to have children with.
So to us, secondary doesn't mean that there are limits on days together or physical acts or feelings anything like that- it's a simple acknowledgement of the fact that my husband and I have made a lifetime commitment to each other with entanglements. If there are times in my life where prioritizing is necessary, he's going to come first.
We don't really do "rules". My husband and I discussed our personal boundaries around safe sex, and how out we wanted to be, etc, but we both acknowledged that when one of us started dating a new partner, that we'd have to sit down with that person and listen to their wants, needs, and boundaries as well. We don't believe in presenting a partner with a list of rules

And we did have that conversation when I started dating Charles.
We discussed how often we wanted to see each other, safe sex protocols, what we were looking for in a relationship, etc. And in our case, a long questionnaire about BDSM and D/s.