Help me

My gf is bisexual and very sexually experienced. She has had 3 ways several times before meeting me, and I have only been with 1 partner at a time. She has asked several times if she can hook up with another woman and I told her only if I can be involved. My opinion is if you fuck someone without your SO that’s cheating.

I understand I can’t give her what another woman can, and I’ve asked her why we can’t have a 3 way with another girl and her response is she would be too jealous. The thing is she is asking to hook up with a girl she previously slept with before we got together, and I told her it sounds like you have feelings for her since you’re against us doing something together rather than alone. I can’t imagine having sex with anyone without her so it hurts to hear she wants to hook up with another woman alone, and not only that but someone she has emotions towards. She told me there’s no threat and the emotions are more friendly than loving which I believe but I don’t want my gf doing things alone, especially when there’s an emotional connection. Why can’t we do these things together?