I am bisexual and married. If the two of you don't have an agreement where you've said it's okay for her to have sex with women, then it's cheating.
I don't think you should be mad that she's bisexual, but I do think that if she wants to have sex with women, and you're not okay with this, then she should break up with you.
I would pose the question to her this way, "Would you be okay with me having sex with another woman?" If she says, "No", then let her know that this is how YOU feel. If she says, "Yes", well then, you have a decision to make --- are you okay with having an open relationship or participating in threesomes, etc.?
More often than not, one of the partners involved in an open relationship is less enthusiastic than the other. It can lead to a lot of stress on the relationship. Sounds like she is not the girl for you. I agree that this could be "just a phase", but if she's truly bisexual, she won't "grow out of it". You can be attracted to both sexes but still practice monogamy.