

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.
If I start dating someone and they never mention that they’re poly, I just assume they aren’t and keep the relationship monogamous. I don’t really like it when it’s opened up, either. It’s hard to tell if someone agrees just because they like me or if they actually want to open it up.
That being said, if someone cheats on me in a monogamous relationship, I’m mostly pissed because I would have said it was fine if I had known that’s what they wanted. That being said, it goes both ways so they would also have to understand that me being okay with them being with other people means I can be too. My first poly relationship was with an ex who only opened the relationship up for herself and got pissed off if I even jokingly kissed my friends.