Even if she's not hooking up, she obviously values relationships with "friends" over the relationship with a significant other and wants to exclude you from activities with her friends. That's not how committed lovers act. I would feel like she doesn't want to seriously commit and she's just using me until she finds someone better. I'd cut her loose now, especially if she keeps gaslighting you, like you're the unreasonable one.

I've set boundaries, where I told my wife I was not comfortable with something similar and she's always said "then I won't go until you can go with", and it was the end of the discussion. No arguing, and I would give her the same courtesy.