What if it was with some other dude she had a fling with before you became a couple ? Just because she's identified as bi doesn't create some biological urge that outstrips a monogamous commitment to a relationship with another. It's like me arguing that I can't be with only one woman because I like more than one colour of hair.

If monogamy is what you need, you have the right to ask for that. You should be just as brutally honest. Do not let the labels take priority over your wants and needs, especially if it's just going to lead to anxiety.
 

Welcome - I agree with Daryl. Bi or not, if someone is opting out of the relationship, or the terms you had when you started the relationship, then you should consider that as part of your evaluation. I cant tell you what to do, but I do know about holding out hope for something that may or may not happen. It wasnt worth it, and likely isnt for you if you are feeling like this right now. Everytime she goes out with her friends that are girls, are they friends, or lovers? You won't know. Not because she is bi, but because it appears she has no problem putting what she wants, over your emotional well being. 

Good luck, hang in there. Do what you feel is best. I am days from finalizing a divorce post 23 years...I dont even think my ex is gay even though she is with a woman now. Ultimately, it does not matter. She had an affair, lied and continued lying for months and did not care about our kids or how bad I felt. She only cared about herself. Your situation is yours...but I think you'll find many on this board with consistent stories, unfortunately.