Sigh... Another thing I really doubt I'll ever get- a throuple of myself, a femboy and a tattooed anarchist punk with green hair, who's just rlly cooooool.
There are a lot of different types of poly relationship structures and different names for them. The base unit of relationship is a standard couple where 2 people are together. Add another person in and they can either be in a relationship with only one of those people and form a "hinge" aka "V" or be in a relationship with both of those people and form a "triad" aka "throuple". As many people as those involved consent to can be added this way.
Most of the time it's one person who is in a relationship with multiple people who are each in relationships with multiple people. This forms a "polycule". Where you have the people you're in relationships with aka your "paramours" and they have the people they're in relationships with aka your "metamours". This group of relationships can take many forms and can be drawn out into a cool diagram like a molecule, hence the name polycule.
The people you're in a relationship with can break up with you like in any other relationship and vice versa. It's more complicated when you add in housing situations if you're all living together, multiple people are all dating each other, or if two people are married.
Using one of my breakups as an example:
I've been in a triad where one person broke up with the other. I was then put in the middle of their breakup drama. I set a boundary of not wanting to deal with their drama/shit talking of the other. One of them kept breaking that boundary, so I broke up with that person while still being in a relationship with the other. Luckily I was living with the person I stayed with or that would've been way more complicated.
in case anybody who doesn't know, poly doesn't mean everyone is dating each other. Someone in a poly relationship can date someone who has no interest in dating their other partners. ofc a good rule of thumb is that everyone in this metaphorical web should be able to sit down and have dinner with each other without being mean or violent with each other.
While this is certainly a valid form of romance, it's more accurately described as "non-exclusive simultaneous relationships" than a single "polyamorous relationship".
Some people really do live in multi-partner committed households, but those seem most often to be dominated by a single person, such as fringe Mormon polygamy. And the most common form of "polyamory' is probably "affair-tolerant monogamy."
It's a big complicated world, and variations of how humans with form intimate relationships fills all possibilities when there is no enforced legal prohibition. (And,.sometimes, even then.)
As a poly person: no, it is not a "affiar-tolerant monogamy". That is an open relationship.
Polyamorous partnerships are far more committed. Also, sex is not always a part of it.
Of course there is the concept of a primary partner, but there are lot of poly folks that thislike this idea.
But what all of those relationships have in common: there is no case where only one partner is poly. All is about communication and consent.
And to the core topic: There is this thing like a polycule. A network of people with somehow connected relationships. Breakups in those structures are often consensual and no big fuzz. But if it gets dirty, at least in my experience, the offending member of the polycoule gets shown the door. And most of the times, those are the new ones. People that think the could convince their partner to get monogamous because they are the only one that is needed.
I engaged in the "affair-tolerant monogamy" variant when I was younger. I discovered there's a positive curvilinear relationship between amount of drama and number of romantic partners. I am sometimes barely able to handle my own incidental drama, so it didn't last more than a few years.
It is pretty rare for my partners to date each other, so most breakups are usually “normal“. Even when they do, one breakup only concerns the two people involved, unless something really bad prompted it, which has never happened to me directly.
It's because the (western?) default image of a break-up is a messy one. You don't just "remain friends". You fully cut ties and try not to even think of them until 4am.