I agree with Divine. He was likely being platonically friendly and felt really uncomfortable when you mistook it for possible sexual interest. He may choose to keep his friends and his romantic partners very separate, and felt threatened when you tried to merge them. He may also be extremely private about being poly and got upset when you messaged him through this group, where the other members could see his name or contact info. I'm not poly either, but I would probably have a negative reaction if anyone included me in a group list/mail without my permission. I like to maintain control over who has my contact info and who knows what about me. I also have a really negative reaction when someone I'm friends with makes any kind of romantic advance towards me-- even when I'm technically single/available. I just like to keep those things separate.