Awh.
Well.
Dating someone polyamorous for the first time is always weird. I remember thinking the same thing. And the truth was that this person was cheating because polyamory is built on trust, egalitarian, and consent. This person hadn't asked for my consent.
After talking to more people who were polyamorous, i grew braincells and realised now everyone is like that.
I became polyamorous myself. It's just the ability to love multiple people at once, equally.
Think of it like this. Your girlfriend trusts you to trust her to have multiple relationships. You are good enough. Beyond good enough.
If you have a problem, talk to her about it. Maybe she'll work something.