You are 21 ?
She is young too ?
Youth is for making mistakes, and learning and growing from them,....
She knows you are interested in polyamory, you also gave her 'your' permission about other men, (albeit, hoping she wouldnt give herself permission).....
In that context, I would drop the angry-routine. Sooo you didnt get to control how it came about. So what ? Thats what you seem really pissed off about. You didnt get to control how it evolved. I`ll guess you are also a bit frustrated, because you have been mono 'for her', and didnt get to be your polyamorous self. Yes, she did do something stupid. I wouldnt make irreversable decisions about that right now though.
Use the experiment/situation to show her how its possible to be in love in one place, and still be interested in another place. You have a opportunity here, to broaden her horizons. It`s up to you to seize that opportunity, or to chose to wallow in being upset about it.
But do this all in a few days time. You need to just vent right now (which you are doing) let yourself feel pissy, ( without attacking anyone verbally) and give yourself some time to mull it over. In a few days, you might see the glass as half full, not half empty. If I were you, I wouldnt react right now,...just give it time, and you might see various perspectives on this.