Hi 709love and Penny M, welcome!
Let me offer a GG's way of looking at things.
I understand your confusion. But ultimately, it doesn't matter what labels your boyfriends are … gay, bi, hetero, or alternate hetero depending on how the genetic male they are attracted to is dressed or identifies. What matters is that your bfs have the potential to be attracted to persons who offer an appendage that you cannot compete against. And this can be cause for some anxiety.
It's true that most of us (unless we are into open relationships) expect monogamy. But even in traditional hetero committed relationships, there is always a risk, if even slight, for a straying eye - especially when the newness of a relationship wears off. What woman hasn't been to a party with her SO and checked out the competition?
But, it's easy if we only need to compete against other women. After all, we have the same equipment and the same ability to enhance it if we want to. It's another matter if the object of attraction has something that we physically cannot have. Not that our SOs would necessarily go for it, but if they have sexual thoughts about other women we can bring our SOs home and make them forget that. lol. Not so if the thoughts go to sexual possibilities with people who have what we have not.
So yeah, I sympathize with your concerns. The potential to be sexually attracted to multiple sex/gender combos seems to be (according to many threads in this forum) rather prevalent among the CDing community. This is simply one aspect of baggage we do have with gender fluid SOs that we don't have with non-CDers. There can be a "is he or isn't he attracted to someone who has stuff I can't compete with" concern niggling at the back of our minds.
The best advice I have is to talk to your SOs if you are feeling at all insecure. Above all, also be secure in yourselves and in your abilities to attract your SOs. I have personally weighed all the above and have decided that the relationship with my SO is worth it. I guess every relationship has its own baggage.