My boyfriend just told me he's been dating someone else behind my back, but it's ok and it's not cheating because he's polyamorous. I don't know the person and from talking to them over the phone they're really annoying. I don't want to lose my boyfriend but also I feel really bad he didn't even tell me he was dating someone else.
What!? Your boyfriend is wrong. Your boyfriend cheated on you, full stop. He doesn't get to tell you what is and isn't okay. If you're not okay with it, it's not okay.
"I took your stuff without asking, but it's not theft because I'm a communist." No, taking people's things without permission is stealing, even if you don't believe in the concept of private property. They also need to be in on the philosophy.
What your boyfriend is doing is not practicing "polyamory," but cheating. Polyamory requires open communication and consent of all parties. Just because he has a fancy word for what he's doing doesn't mean you need to go along with it. Stop talking to his other partner on the phone, stop bending over backwards to accommodate his twisted worldview where he can do whatever he wants without consequences.
Leave this relationship - he cheated on you, acted without your consent, then told you how to feel about it and continued making demands on you. This is not polyamory, it's a person acting cruel and selfish and obnoxious. Dump him.