I used to date my Dungeon Master (Dungeons and Dragons, not anything weird) and one of her friends she grew up with. The relationship was pretty brief, but she was really good at emphasizing communication between all of us. The breakup didn't even really feel like a breakup.
A couple years ago, she died in a car wreck with one of the players in my group and even though we weren't dating at the time, it made it harder for me to get back into the dating scene after the relationship I had around that time ended shortly after. I briefly got back with the other person I dated, but it wasn't the same and they weren't poly anymore.
If I was in a relationship right now and got over it better, I'd definitely be poly. It felt really normal.
I hang out with a lot of poly people despite not dating anyone at the moment, and I know of a relationship where one partner is either doing that intentionally or on accident. They're the only monogamous person (and admitted to me they don't want to date someone who is poly but don't want to break up), and dating someone who has multiple serious partners that they've been with longer. I really can't see it going well if this person doesn't say something, even if it probably would lead to a breakup just due to being incompatible. I feel like it's not my place to warn their partner about it, so I'm just burdened by this information that I don't really want to know about.