Aside from "what a mess" ?
Overall, I am not surprised a couple open to both mfm and ffm are going to have more opportunity for the former. However, there are some deeper red flaggy issues going on here it seems.
The idea she is just banging another dude 1 on 1, and you are considering that a threeesome is just weird. That she gets defensive when you want to walk about the equity of the arrangement is also not a great sign.
If I were in your shoes, I'd seriously be considering a coming to Jesus discussion. Not addressing a lack of fairness or apprehension you are having will blow up in your face eventually.
For instance, there was a point where my wife and I experiemented with solo play. She did not like it one bit, and asked me not to do it anymore. No problem, I cut that type of play out. She is more important than a little fun on the side.