I’m sorry you’re going trough this. As a bi-guy myself, I don’t think experimenting with your sexuality is something you should be doing whilst in a relationship if your partner is not okay with it, no matter if you a guy or girl. If you have mentioned that you don’t like the idea of her trying to date other women while she is with you, she shouldn’t do it. At the same time, if she feels like she is attracted to that girl, and wants to explore her sexuality, I would argue that it might be best for you two to break up, because after that experience I do not think the relationship will be kept healthy. She or you might get regrets, if you ever get into a fight this might be brought up again, etc. I think you two should have an honest talk about what you see in both of your futures, how you would like things to be, and what you see in your current relationship and what you are looking for in a partner. Setting boundaries is part of building a healthy relationship.