I am the mono GF of a "binogamous" woman, who has a BF of a couple of years. While not married (and not gonna be), they do have a commitment to each other. Being the newcomer to this situation has its challenges, because they have a head start of intimacy, which is something I want in my life.
We are also challenged by distance, which, I think, is also a good thing for us, since my GF and I are both passionate and totally gaga for each other.
Admittedly, I have had moments when I have considered dating another woman locally. But, in all honestly, I am so in love with my GF that another woman in my life would just be a stand-in for when I am not able to see her. And that just seems wrong to me.
I am finding that I have to be careful negotiating this emotional space that I am drawn to when we are unable to be together. This place says to me: "Well, she has someone else. You can too." While I know that to be a potential truth in the matter, I am also well aware of the fact that my GF has a GF and a BF because that is how she is. The same is not necessarily true for me. Still, I have not completely closed the door on that option.
All this to say, I know this place of confusion. I get through it by focusing on the immense love and support I have in this relationship.
Stay strong and true.