OK, ED is not something I have a lot of experience with, but perhaps i can tackle this form the aspect of male ego.

Although he *knows* that his problem is medical, there is a part of him that will feel less than a man because he cannot function with you. He feels that you view him the same way, and your inablity to do what he wants of you only increases this view. Your natural aversion to servicing him causes a negative feedback loop. He feels insecure, needs reassurance from you. You serve him, but he can tell it is out of 'duty.' He feels more insecure, you feel more like you are being used as an object than a person. etc.

I think you can see where this is going, you want to talk about your feelings, but have you spoken with your Dom about his?

You wonder why it has to be about his penis, the only answer I can give you is because it is. By you sucking him off, even if he does not get hard, you are showing him that he is still your Dom.