Reading all these stories about unhappy, non-poly partners allowing themselves to be cucked is hard to understand from an outside perspective, but as someone that narrowly dodged a bullet, I can see how these guys (and occasionally girls) get manipulated by people they thought they loved. Poly people, and the poly community in general, pull some devious, evil shit to get their boyfriends and girlfriends to play along with their cult-like fantasy.
Storytime: I was dating a chick who, after two fucking years, asked me how I felt about opening up the relationship. I said fuck no, I'm not a Mormon, swinger, cuck, etc. And from then on, I was incredibly emotionally manipulated by her and the poly community. She told me I was confining her, and that if I truly loved her, I would let her experience love to the fullest, "because she had a lot of love to give".
She sent me books, videos, websites, that all told me what a piece of shit I was if I didn't allow myself to be cucked. She even fucking took me to a "poly friendly" therapist for the specific purpose of turning me poly. She and her friends fucking sent me "poly memes" every day, it was a complete fucking indoctrination into a cult. And this was someone I thought I loved, and cared about me. I genuinely was gonna reach my breaking point, and I was convinced I was a bad person for feeling repulsed at the idea of "sharing" my girlfriend.
Luckily I found boards like this, where sane people mercilessly ridicule the propaganda I was exposed to every day. That's what helped me get out. Don't be fooled by the Tumblr-grade wokescolding and utter stupidity. Polyamory is a cult and these people are all buying the bullshit. Swear to god this shit is gonna kill someone someday.