No, I've actually only ever had two partners who were monoamorous by default. The first two. Everyone afterwards immediately knew I was poly without having to be told. And was poly too. I mean I would have asked them to consent to being metamours with all my existing partners anyway so definitely no cheating since you brought that weird point up. But if you're now deciding the point is people's assumptions, everyone I know assumes people are poly. If someone in my circles is monoam they actually have it listed in their bio so everyone knows not to flirt with them.
Sounds like I just have gayer friends than you do and you're assuming everyone is like your boring friends.
Do you even have any otherkin friends?
Shel'kek nem'ron!