Of course you are not wrong to be concerned about your boyfriend staying friends with his ex. Your needs are very important in this and if your boyfriend is turning a deaf ear to your feelings then you will inevitably begin to resent him.
It is all too easy for people like your boyfriend to take the line "we are just good friends now" or "you are so possessive and insecure".
The reality is that your boyfriend probably feels like less of a "bad guy" for leaving and upsetting his ex. Does he worry about pleasing people?
For all of us, letting go of someone is painful, even if it is our choice. Staying "good friends" means you don't have to miss them so much. Chances are that this friendship is also painful for his ex and you already know she didn't want to lose him and tried to get him back when he was starting to see you. Seems likely she is still hoping she has a chance.