I think we are agreeing on this for the most part.."A man is monogamous only when he has just one lover or spouse" This is in fact my case.
To be clear this is the dynamic of my mono/poly relationship. I have and want only want one lover. I am not open to sharing that intimate love for anyone else but her because that is how I love. My lover is polyamorous and intimately loves more people than me. She has the personal choice to consider adding more.
It does mean that I will never date outside of this relationship unless the intimate nature of it changed...that's a part of my monogamous nature.
I too had similar concerns about the addition of new lovers and the uncertainty of wondering if any number of lovers would ever be enough for her.
I have chosen not to worry about this issue any more and just enjoy her to the fullest, with abandon. This took a lot of communication and allowing myself to be vulnerable.
If I continued to play the "what if" game or saw a new relationship forming around every corner, I could not stay in this relationship because I would honestly go insane LOL!! I did feel comforted when my lover told me there actually was a cap..I'm not saying I am it but it felt good regardless.
There are things she does not have and I understand those and want her to have them....I would step back if that is what it took to make her happy to the fullest extent.
I know how important I am; she knows how import she is, we both know how fortunate we are..all of us, everyone in her and my life.