Thanks for responding.
Based on the conversation we had when we broke up, I don't think he was actually poly friendly or accepting, which shocked me because I didn't think a mono person could get involved with a (honest, open) poly person if they were actually against polyamory. I think what happened is once he developed feelings for me he tried to pretend I wasn't married, because he didn't understand enough about poly to process it. That obviously didn't work too well, and by the time I got out of him what he was feeling he was just really bitter about the situation, and he was long past the time we could have worked it out. I just never knew how he was feeling, and I feel confused and like an idiot for not. I try to tell myself that they're only so much I could do, I'm not a mind reader and I did try to talk to him about how he felt, he just always had something else that was getting him down and not me. But then I wonder how I'd prevent that from happening in the future? Maybe I'll date only married people from now on. ;-)