First time responding to a post on here, as I am new to this as well, but definitely not new to the relationship "game". Listen to the others on here. Your BF may be truly poly, but he's definitely not treating you with respect. The one thing I've come to realize is that the incredible beauty of being truly poly is that you have to communicate - frequently, thoroughly, and with love and open-minded ness. My BF is not new to this as his wife has been open and dating for a while, but I am his first GF and I am learning so much about how this should be done. My husband is not poly and it's been extremely difficult for him to make the transition from monogamous to me being poly amorous, but I am so, so careful to treat him with respect, love, and not force anything on him that he's not ready for.
Your BF is not giving you the same consideration, and you deserve it. It seems like he's forcing the idea of taking on another girlfriend on you, and adopting a "like it or leave it" attitude. Meanwhile, he's enjoying your hospitality and your paycheck. He's got it all! Why should he leave what is, to him, an ideal situation?
Honey, it doesn't matter if he's poly or mono - he's disrespecting you and hurting you. Turning the tables and blaming it on you (calling you "territorial", for example, when you object to him dating a roommate!) is a manipulation tactic. It is not your fault that you do not want to share him. It is his fault for forcing the idea on you and then making you feel guilty for not agreeing to it. You should probably take a break from this, remove yourself from this situation to get a clear head, and then ask yourself if this is how you wish to be treated by someone you love.
Btw, just because someone dates multiple people does not make him poly amorous. Being poly is the capacity to love more than one, not just want to date or f*** more than one. Is he truly poly? Or does he "want his Kate and Edith,too"? Either way, it sounds as if he's found a way to manipulate you into getting what he wants, and your needs are not being met. People who truly love each other treat each other with respect. Good luck!
"Being deeply loved by someone gives you strength, while loving someone deeply gives you courage." ~ Lao Tzu