You answered yourself already. You do not like the sound of your GF being with other guys because
a) currently too jealous and insecure to agree to it.
b) prefer to share female lovers (because that feels good/safer?) You've shared good sexual experiences there with GF and female lovers so it's not as scary.
Could read more on jealousy. Dig into the WHY of your own jealous. And why her polydating "women" feels safer to you than "men." To help resolve your upset.
Could think on whether UNSHARED female lovers could upset you too. If so, maybe it isn't the gender of the person.
Could think on what makes it scary in other ways. It's fear of demotion? displacement? intrustion? Something else?
It's about competition, confidence/insecure stuff because you live with a bullying inner voice?
Is it required or expected that an open or poly relationship be open to all? Am I just not cut out for polyamory?
Even if jealousy and insecure is resolved and you are totally solid and secure in yourself?
No. It is not a "requirement" that if you are in an open or poly relationship with your GF that you "just be" agreeable to her being with all people and YOU concurrently. She can have her preferences. But so can you.
You have in internal list of "Messy people" I am sure. I'm also pretty sure people like your boss or your parents are near the top of the list! And you would NOT be eager to participate in a polyship arrangement with her and those people and you
She can always date who she wants to. You do not control her. But she cannot date X and you
concurrently without YOUR goodwill and willingness to participate and have is result in a harmonious polyship.
Because you control you and what you wish to be a participant in or not. You can always say "No, thank you. Not like this. I do not want to participate in that. Sounds too messy for me."
Your preferences for your romances (for whatever reason) are your preferences. If you were into monogamous arrangements and offering her a monoship with you, she could sign up to participate or decline to participate.
If your preferences for an open model
presents her with an offer she digs, she digs it. If your offer is not something she's into, she can decline.
So just spit it out.
"My preferences at this time are _________. It could become _______ in time but no guarantee. That's the offer on the table. What's your preferences?"
See if you are still compatible and still wanting the same things at this time or not. Even in the same person, what they might want from polyship could change over time. What you want as college students may not be what you want after grad for instance.
How about telling GF you are willing to talk about it and sort it out (assuming you really are) but NOT willing to just jump in with her polydating men until you sort out what your jealous/insecure is pinging from inside you and what support/nurture you may need from her in that process.
Could she be willing to consider postponing dating while helping you sort out what it is you need from yourself and from her, all that, or is she not willing? This article
while written from the POV of going "monogamous ---> Open" delineates steps to a possible approach. Just as easily could be you guys going from "open with women ---> open with both men and women."
Everyone could state where their willingness lies at this time and sort what they are willing to do/not do. Then sort it out between you.