Well what he has with his girlfriend is not poly in my belief. It is open however. That could be a problem. It doesn't sound like she is willing to face all the emotion and inward discovering that poly entail. Of course I'm sure she has had to face some in order to get as far as being open.
I haven't really gotten how that all works when someone thinks that if they don't know what their partner is doing then they won't be replaced. I would replace a partner much more quickly if I wasn't letting them in on what I hold near and dear. The closeness and connection wouldn't be there and for me that is a deal breaker.
I have a bit of a red flag about the guy. I get a bit wary about people who just say, "don't worry about it." it kind of makes me think that they think they can control their feelings and mine in doing so. It spells disaster to me as at sometime, if that is the case, then that energy needs releasing. No one can control anyone or their own feelings in situations you speak of forever. At least as far as my experience has been.
I think that it might be best to tell him how you feel and go from there. He might not want to jeopardise what he has with her just because you love him. He might think this deal he has is better than admitting loving you. Or on the other hand perhaps you are the one that will be worth it enough to make some changes to his primary relationship. You won't know until you start honestly communicating.
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