I don’t know how I know so many poly people but I do and they’re always popcorn fodder when the inevitable goes down.
This is the poly relationship cycle as I’ve witnessed it across several poly groupings (mind you, all parents with kids):
Step 1: new shiny plaything is introduced as girlfriend/boyfriend. Plaything is MUCH younger than established couple and so excited by all the attention.
Step 2: group dates take place, everyone is SO HAPPY and proud of how well everything is working out.
Step 3: plaything moves in and becomes bonus mom/dad to children in the home. Everyone is SO HAPPY and kids love their new auntie/uncle.
Step 4: jealousies and tensions arise. One of the establish couple feels jealous. There are many Talks of feelings and ways to address feeling left out.
Step 5: resentments summer but everyone is still SO HAPPY
Step 6: plaything disappears and all evidence is wiped from existence. No one speaks of them ever again.
Step 6 (alt and most common): everything goes down in a very public, colorful, and noisy fashion. Established couple accuses plaything of using them for money/housing, changes the locks, and posts emotive memes on Facebook about betrayal. Plaything outs established couple as manipulative assholes who thought they’d gotten themselves a new house slave/nanny/sex slave and they’re filthy people to boot. Sides are picked, lines are drawn, and it gets nasty. Eventually we bring up this story around the backyard campfire when attempting to one up another person’s story of the depths of human depravity.
Step 7: lather, rinse, and repeat
I’ve seen this play out time and again. Sometimes with added drama (pregnancy, job/career loss) but in the end it comes down to a ruined relationship and kids wondering where their auntie/uncle disappeared off to.