Well this goes back to you being sexually imcompatible. She's also put restrictions on you wherein you can't touch the other girl, only she can. It's all about her. Some are okay with watching their partner have sex with another girl, or guy for that matter but it appears that you are not so now that you've talked about it and she is bothered that you don't agree it comes down to plain trust in that she won't go ahead and experiment without you.
... Are there any rules that if you changed up, you'd be okay with it? For instance that you do get to touch the other girl and you are the center of attention as much as she is for instance? Or is the whole idea of it just not something you want to partake in no matter what the rules/compromises are?