It’s ok for her to want an open relationship and explore her bisexuality.

It’s ok for you to not want this at all and stay in a monogamous relationship.

Nobody is wrong here. But clearly you too are no longer compatible when it comes to a relationship. No matter which route you go, someone is going to feel bitter about it.

Seems like it’s time to break up and move on and find more compatible partners for both of you. You’re very young. If you’re not happy and it’s not working, it’s ok to walk away.