Poly or Reverse harem?

She says we're in a polyamorous relationship. So why does she always refuse whenever one of us wants to bring another woman into it? That's the question I've started asking myself. Amy has always called what we have polyamory, and for a long time, I believed her. But the more I learned about it, the less it made sense. Polyamory means being open to multiple relationships, doesn't it? Yet every time Adam, Sora, or I mention the possibility of seeing someone else, Amy immediately shuts it down. The strangest part is that Adam, Sora, and I aren't even lovers. We're friends. Good friends, sure. We live together, spend time together, and care about each other. But there's no romance between us. The only person any of us is dating is Amy. So if we're really in a polyamorous relationship... why does it feel like Amy is the only one allowed to be poly?