This was a very obvious power play/assurance seeking ploy on her part. When faced with those, I've found it best to reword the test into basic, but somewhat ambiguous terms that lead to a completely reasonable, emotionally neutral result of her choice to speak or act in a certain way towards her "exclusive" partner that she probably doesn't want.
"I see, well maybe we were too quick to become sexually exclusive if you are even considering experimenting like this with other guys. We can step it back to just dating and having fun together and see where things go, sure."
"I see, well maybe we were too quick to become sexually exclusive if you are still (responding this way to, encouraging, looking for, feel the need to declare to me) attention from other guys. We can step it back to just dating and having fun together and see where things go, sure."
You may hear a variation of "Are you breaking up with me?" and the answer is "breaking up is such a harsh term, if sex or sexual attention from other men is important enough for you to bring it up, then we just need to slow things down or take a couple of steps back from exclusivity, no need for any extremes unless that's what you are asking for."
If you get, "is that what YOU want?" then the response is "I'm happy to stay exclusive for the time being, but I don't feel the need to have discussions about sex with other women or sexual attention from other women with you. If that changes, though, you'll be the first to know."
The key is the clear communication of a strong presumption that "exclusivity carries consequences and there are clear boundaries that are beyond discussion if we are going to remain exclusive," such tests have instant, real results that she may not have intended. They should stop, has usually worked for me. On the chance that she is trying to bait a breakup, this just gets you out and moving on faster, so it works both ways.