To me, marriage is about giving %100 of myself body & heart. I can without a doubt say that my husband has everything of me. And I believe I have %100 of him. I dont think we would be married If I thought anything less. With that being said I think I was in your reverse position. I demanded that My husband be sure I have %100 and there is absolutly no remove for his ex even as a friend. I cant say its right, buts its how I feel and am and it was for my husband to decide if he wanted it or not..... I guess he did cause were married.
I know this is hard for you, I had a hard time when I was the one in your FI position, thinking if I was being insecure, or unreasonable. But no matter how much time I myself gave, or tried to see a different side, my feelings on the position never changed. And I made sure my husband knew how I felt and what my expectations were. So, I think its up to you, are you willing to let go of that part of your life to move foward with your FI? Or do you think his request is unreasonable and he can either be more secure in your relationship as it is. I dont think either are wrong, but I do think both are very hard. But thats what marriage and relationships are all about.
I wish all the best for you and your FI. What is meant to be will be.