Wow, no, I would say this is not what it's "supposed to be like", and certainly it's not what most people do. He says he needs to be the most important person in his own life, but why does that mean you can't be important enough to share things with too? How can you feel secure with someone when you have no real idea of what's going on with them? It doesn't seem like a healthy way to live, for him or for you.
His sexual and emotional choices affect you too, because of your level of intimacy with him. If he absolutely won't let you have a window onto that part of his life, I think its perfectly valid for you to feel like you can't be in a serious relationship with him. There is a middle ground between dependency and walling yourself off, it seems bizarre that he doesn't seem to realize that.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.