Relationships are interactions and connections between people, no one has a relationship with an existing relationship. It is possible to develop a relationship amongst multiple people, or for someone to develop multiple relationships with people who have relationships to each other. But, whatever the connections are, the existing relationship is still there and between those same people, what is new is each of their relationships with another person (or each of their relationships to other people if they are not in a relationship with the same person) and possibly an additional inter-relationship as a group (because group dynamics are very different than the one on one dynamics of the people involved).
People who want to play "poly by monogamy rules" inevitably treat their relationships outside of "the one" poorly, sometimes outright abusively. If you have multiple partners, and are only honest, forthcoming, and truthful with one of them, what kind of jerk are you to your other (most likely in your mind "lesser") partners? If all of your rules were set in advance with only one person, where is the true consent and participation in your relationships with other partners?
It's true, some people may buy into being "secondary" partners, and accept all of the previously-negotiated-with-only-one-person rules in advance. They will likely try very hard to be OK with rules being imposed on them from outside their actual relationship. In the end, the fact that a person outside the actual relationship has control or discretion over the mechanics and actions within the relationship will cause a breakdown in the relationships (and it unsurprisingly may not go the way you hoped when you created all the rules to "protect the existing relationship"). To break it down a little simpler, if your new partners have no say in their relationship with you but your existing/other partner does, hurt and pain is coming.
I realize, that a lot of people choose to be assholes in this life, and therefore have no problem with treating people they profess to love and care for this way. Outside of removing people like that from the gene pool, what the rest of us can do is:
1) Try very hard to teach them how to treat people well, and as real human beings instead of fantasy/wish/unmet-need fullfillment objects.
2) Convince the people in relationships with this sort of person that they are valuable and deserve partners that actually treat them as partners, that they can have ability to participate in the structure of their relationships (all of their relationships, not just one.)
3) Stop, simply stop accepting the mono-normative/couple privileged bullshit that these people spout. I don't care if they set themselves up to be 'poly experts' or claim years/decades of poly experience, if they've published a book, written a blog, or woke up yesterday with an epiphany about the only way for multiple relationships to "work", if they cling to the only possible starting place for functional polyamory being "protect our existing relationship from any change" make sure to lock the door after tossing them out on their rear.
If someone is deathly afraid of change, they aren't really capable of trying a different way to have relationships. If they must have the illusion of monogamy at all times, they aren't ready for polyamory. If all they really want is exactly what they have exactly the way they have it, and perhaps a little something on the side... tell them to piss on their own side of the fence.
I'm not saying that having strong loving existing relationships prevents people from being polyamorous, or treating other partners well. It's possible to do that, what's not possible is preventing any relationship from changing when new elements are added to the mix. There will be new facets to your existing relationships, some awesome and some painful, based on how you change from your experiences and interactions with other people. New people are not part of your relationship but they will change it, often in ways you haven't expected.
Poly is a way that we as people, as human beings, have relationships. It is something that people do, not couples/relationships do. Existing relationships consist of multiple people, each of them has to choose for themselves to have polyamorous relationships.
Poly is not… I repeat: POLY IS NOT something your existing relationship does, it's not about "couples".