It is not solved, it would be a compromise for me. I don't know if I would be resentful but there is at least a chance that I would not be.
If I agree on something, then the blame is on me for agreeing.
I have no problem with the gender restriction if she's fine with it. It depends on her motivation. If she, in fact, wants to date guys too, then I'm not sure it will work.
But I'm not sure 'compromise' without deep understanding and acceptance is enough here.
I see you're taking on responsibility for making the agreement. That's nice, and it helps conflict resolution later. But if you DO become resentful, then what? Do you expect yourself to suck it up? And if she can't handle the first time you're with another girl and she can't handle it, do you expect her to suck it up?
I hope not.
Poly doesn't work under such a rigid mindset. Every rigid unchangeable agreement you make will come back to hunt you - just as your monogamous agreements aren't working for your girlfriend now.
What I encourage you to do instead is baby steps.
Maybe your "open just for girls" agreement will work for you eventually.
But for now, go slow and commit to doing inner work as you go on - both of you. Try to understand with an open mind what she's missing in a monogamous relationship. Read up on open relationships more. Visit a poly meeting and a swinger's club and just talk to the people there, see what good stuff they get out of being open. Go to a kink event, if that's your thing. Invite a girl out just for dinner. Let your gf snuggle with a friend (if they are up for it), and just see if anything about your relationship changed. Disentangle
a bit, think about granting each other more independence. Examine your beliefs about sex and why sexual exclusivity is needed.
Keep a goal in mind (like partially open with both people happy), and work towards it, always just stretching as far as possible at the moment.