Echoing the above statements regarding her being bi and you being a man. Personally, well before I identified as Poly, I have dated MANY bisexual women, and have never taken issue with them having girls on the side, even serious relationships. Not criticizing you, just hoping my POV will rub off a little and help you. I will explain it the same waay I explain it to my friends for years now whenever they ask why I let my girls date girls.
All things Poly aside, you're a man. You look like a male, smell like a male, have male habits and traits and body parts. You're straight (I assume, which is bad, but if you were bi you might not feel quite as confuddled as you do). How do you know you are straight? Well, because you don't like all those things I just listed bout men. You like all the things that are different about girls. You like female looks, smells, habits and traits and body parts when seeking a partner. You can't get that from a man - that's why you're straight.
Your girl likes what she gets from you. That's why she's with you, one would hope! She also like the same stuff that you do about girls. You cannot offer her that. It;s no threat to you in any way - relationships between two women are VERY different emotionally, physically, mentally - all the way around, this other girl would in no way be a replacement for you - she can't be, any more than you can be one for her.
In every relationship I've had where y girls dated girls, it's been nothing but positive. Either she has a seperate thing with the other gal and you get a little more you time to do anly things, or the relationship with her includes or involves you - use your imagination, it's everything you can imagine andmore, lol. Either way, it's a win.
A few points to follow if you can handle her dating women:
1. The only "demand" I ever make is that if the other girl tries to interfere with our relationship, she's history. If I can share so can she.
2. Don't psh her or bug her about her girlfriend. No asking for threesomes if you think she's hot or whatever - if they want you to play they'll ask, believe me, and if it's your idea and not theirs they'll either resent you for it and it'll NEVER happen, or it'll happen and be awkward and no fun and cause issues down the road. Same for things non-sexual. Don't qustion or pester her about her relationship in any way - if your input is wanted - and sometimes it will be - it will be asked for.
3. Understand that there will be NRE between the two of them for the first little while, and don't be bothered by it. They will spend a lot of time together and do annoying girl things like whisper and laugh a lot if you're in the room. Don't take it wrong, it's almost always a good thing in the end. Don't feel ignored or unwaanted during this phase when you get a little less attention. The better you handle the first few weeks, the more understanding you are in their eyes and that appreciation will come back in wonderful ways you cannot imagine. Not even sexually necessarily, just in the form of a vary greatful girlfriend who thinks even more of you than she did before becuase you "get her" so well and "let her be herslf", etc. It's a good thing.
Spend some time and wrap your head around it. You can do it, I promise. It could be worse - whe your GF tells you she's poly and loves another MAN but doesn't want to lose you... Well, talk to a few of the guys on here about what it can take to wrap your primitive and posessive man-brain around THAT one, lol.