In that case, substitute "In a poly relationship" for each instance of "poly" in my previous post. Yes, there are many mono people happily involved in poly relationships. It doesn't sound like you'd be one of them.
Reading your reply to GG doesn't clarify the situation—assuming by "the situation" you mean his failure to spell it out in words of two syllables or less that the conversation about how he wants a poly relationship, followed by the two of you getting romantic, meant that he thought he'd conveyed the fact that a relationship with him would be a poly one. It says to me that you were hearing only what you wanted to hear. I don't think he could have spelled it out any more clearly.
Effective communication requires that the listener actually listens, not just that the speaker is clear in their words.
He absolutely could have spelled it out more clearly. We spent one romantic weekend together and for all I knew, that was going to be it. We talked about polyamory in theoretical terms, as it concerned his life, not mine. He never asked me how I felt about it. He never asked me if I would be interested. What part of that is perfectly clear? I hear so much on this forum about communication and being honest and straightforward, and that is not what happened here.