At first I was just going to offer virtual *hugs* and link to this thread
because I thought it might help to know that others have had the problem of "I want to be with X but they aren't poly!" I totally understand that not knowing what people are going to discover about themselves and what decisions they're going to make is scary, but one thing you might want to try to keep in mind is that yes, he could decide he's mono but choose YOU, rather than D (even if he's mono and only wants one relationship for himself he could still be supportive of/okay with you being poly!). You have a chance even if it turns out he's NOT poly, so it kind of goes back to dating rules that always apply: someone you want to be with might not want to be with you, and that can suck, but that's the way it is regardless of whether someone is mono or poly. *more hugs* It does seem like there's a LOT going on with you all right now, lots of different directions things can go, and that's unsettling even if it's exciting. I hope everyone continues to handle it all well, with honesty and communication!