It is difficult to answer. Sometimes it gonna be dangerous to keep such relationship with your former boyfriend.
I just finished a such kind relationship today. I think I can keep friendship with my former boyfriend, because we all married. But he always want more than you can give to him, you want to treat him just as a normal friend, but he don't. He is such a self-concern person.
You see, you used to have intimate relastionship with that person, how can you keep pure friendship with him or her?
So that's my falt to see him as a friend, but now I did a right thing to over it.