Before you get too much deeper then, renegotiate the terms.
It's perfectly valid to go into a thing and make self discoveries that could change the name of the game. We can't anticipate everything! But the thing to do then is go back to the negotiation table and work out the next working game plan for THIS stage.
Sounds like initially you were looking for something like a "closed polyfi V" kinda of situation with your BF as the hinge person with the arms of the V not having anyone but him and him having just you two. And with your crush on the OSO, hoping it could extend to triad perhaps.
And now BF wants it morphing into something like he wants to have you two but not be closed in his end, yet you all remain mono-closed to him. If that is what you all want and it's your pleasure, yay. If you all have not signed off on that structure for this next stage... he is being very cavalier and presumptuous with his people and their own wants, needs, limits.
But he did say that he hopes that I don't get mad because he now gets to be with other guys, and I have to stay monogamous.
He doesn't just "get" to be with other guys. He wants this, it may not be yours and it needs to be negotiated.
You do not "have" to stay monogamous. This is his want. It may not be yours.
He "hopes" you don't get mad? He may actually hope that. But it is ridiculous to hope someone would be thrilled to have their life dictated to them even down to how they should be feeling!
Or more sinister -- he's not hoping. It's also another order. He gets to be with other guys, you have to stay monogamous, and you cannot be upset by all that. Stat!