I'm friends with one of my exes, but the relationship was during our teen years when I was very confused about my sexuality. Now it's pretty much a distant memory that we kinda laugh about.
In all of my relationships except that one, I've been the dumpee. And all of my other exes have eventually come back to try to reconcile, and I inevitably rejected them. Later on, we'd try friendship, but there was a lot of animosity in most of them because I rejected them (ha, and yet they dumped me), so we just couldn't. I've since given up on the notion. It sounds weird, but I see this as a common trend among all of my exes since the first.
It's sad realizing that someone you once loved so much is just a stranger now. I hope one day I can at least be acquaintances with most of my exes because I am a very caring and forgiving person, but for now it's just not healthy for me. Even an ex from 4 years ago thinks she can make calls on my life and what I do wrong. We recently tried to be friends once more, but I had to distance myself again.