My belief is that an unevenly open relationship carries the presumption of coercion and dysfunction, and only through enthusiastic consent of the constricted party can such a relationship be healthy and happy.
Your attempt to justify this based on your personality differences fails for multiple reasons:
(1) People change - please read this sub, people change thier wants all the time, especially when it comes to interpersonal relationships. If your bf decided that he wanted more another partner, you have no right to dictate his personality and lifestyle preferences. It seems he already has, and you are grasping for an argument that justifies you wanting your cake and eating it too.
(2) You have no idea whether or not you will be able to satisfy your bf once you date other people, even if he doesn't change - your assumption that you will always be enough for your bf once you start seeing other people is myopic and naive. NRE has ruined relationships where the parties involved were well-versed in the risks, yet your cavalier attitude toward his needs (and the potential needs of your future partners) is extremely troubling. Once again, read this sub, hierarchical poly is hard even when you never lose sight of your desire for a hierarchy, which a lot of people do.
This is meant to be more of a wake-up call than a personal attack, but your post makes you come off as a narcissist, and your flimsy justification isn't compelling. A better argument would be to say you are worried that he will have less of his already rare time for you if he had other relationships. However, you phrase it as being unfair to you because he would get more than he needs, a net loss to you. This argument is absurd and belies either narcissism, fear, or both.
Good luck.