Hi Marina! I agree alot with GS. As far as the maturation process (you seem to be in your late 20s?)- men, imho, are not ready for family and added responsibility until their 30s and that seems to be getting later and later. Women have that bio clock ticking which keeps us a little more aware of when things need to happen!
That being said you have alot on your relationship plate and just because you are not at the same place right now, does not mean he will not eventually be there, nor does it mean your goals are different- just out of sync a bit.
It seems your communication skills and intellectual compatibility are there. Everyone needs a break from "The Conversation" whatever that issue may be, in this case deciding whether poly is for you at this time. Don't worry that he needs a respite from the cyclical nature of debate. I am glad to read you are not projecting negative feelings onto C because it is not anyone's fault, it is not a competition and you have your own strengths and attractive qualities. The doubt and insecurity is not because she exists in your life...it is that you are being asked to look at your relationship in a different light. Scary at first. EVERYONE seems to go through this, and you are already on a more open and enlightened path- your circle of people sound as if they could help you through the pitfalls of mono/poly relationships- and everything in between.
But, if you are attracted to C as well and she is part of your network, why not confide in her a bit about your worries? Are you close enough and trusting enough in that relationship to ask her feedback? It may be just what you need, a little reassurance from her and the support of people who really understand. Best of luck. I will be following your thread.