C's desire to seek others is a separate issue from how he feels about you. I think you may be looking at it backwards.
His desire for expansion is not due to a lack in your relationship, but probably rather because he feels safe and secure enough in your relationship to expand. Also, when a person is able to delight in his partners - in your case, you and his wife - it can spur a person to add more to what he already perceives as awesome. It's like I have had two pieces of cake and that makes me want three! Not, "gee those first two pieces were crap, so I'll eat a third."
As it is, it seems C has been pretty clear how much he values you and your relationship, regardless of how you define it.
As for your husband, yes, I can see how you might feel cheated. What do you want from him? What can he do to dissipate your anger? Perhaps when you answer that question, you can discuss it with him.