My girlfriend (21F) is interested in having group sex with other men, but I'm not comfortable with that. What should I do?

I've been in a relationship with my girlfriend for seven years, and she means the world to me. I can't fathom life without her. However, during a conversation about our future yesterday, the topic of sex came up, and she unexpectedly mentioned her desire to have a threesome with another guy. Initially, I thought she was joking since she's usually quite reserved, but it became clear that she was serious. I was taken aback because years ago, we had discussed this, and she had been adamant that she would never consider it. Now, she was expressing how incredible it might feel to be "filled" by other men and even said she fantasized about being "overwhelmed" by multiple guys. She was quite drunk at the time, while I had only sipped a little, as I've never been a fan of drinking. When she fell asleep, I found myself consumed by thoughts of what she had said. The idea of my future wife being with other men while I watched filled me with sadness and anxiety, and I could barely hold back tears as I sat there, my hands shaking. The next day, I asked her if she truly meant what she had said. I tried to approach the subject in an open manner, hoping she would feel comfortable sharing her thoughts. She confessed that she wanted to explore more experiences because we’ve been together for most of our lives, and neither of us has been with anyone else. This made me question whether she was dissatisfied with our sex life. I believe I perform well; we use toys, and she certainly reaches multiple orgasms. Yet here we are. The woman I envisioned as the future mother of my children is now expressing a desire to be with other men, and that’s something I cannot accept. Just last month, I secured a well-paying job and was considering buying her an extravagant ring, but now I’m reconsidering everything. I conveyed my feelings to her, but she seemed taken aback by my reaction. When she brought up the topic again during dinner, I lost my appetite entirely. This situation is incredibly challenging for me. I don’t feel like I have anyone to confide in, so I’m turning to this forum in search of some guidance.