It’s def screwed up to have sex under false pretenses. Not screwed up to have sex with someone and not be sure you want a relationship. But absolutely screwed up if you dangled a possible relationship as a carrot in order to get them to sleep with you. So whether this guys screwed up or not, depends on how much he knew and whether he was intentionally deceptive with you. There is no implicit requirement to turn a sexual encounter into a relationship, but there is an implicit requirement not to be misleading about his intentions.

Generally speaking, I’d say if dating monos looking for something serious, just don’t even say you are poly. If you are down to have a mono relationship, just go out and date for that. Most monos fear of being cheated on is too great for them to see poly people as relationship material. They will see you as someone to have fun with and maybe even build some sense of connection, but they will be saving the “serious relationship” for someone who doesn’t trigger their fears of infidelity.

They may seem very “poly” to you when it’s casual fun, swapping stories of good times and not showing jealousy, even okay to hear that you are dating other people, but the reason they are able to do this is because they are not going to let themselves form a deeper attachment bond.