I can really just be friends with an ex. Though our relationship would necessarily have to change, there is no reason, if all three parties accept the situation, not to keep friends.
I do have an ex, that I am good friends with, that is also my wife's best friend (long story, but I met the friend first). They have insisted that I remain friends with my ex, just so they can continue to have a deep and meaningful friendship. I'm not dealing so well with it though, because I really think we should have parted ways years ago, and I can't stand my ex's husband.
I believe I could just be friends with an ex, but I don't have any still around to test that theory on.
I'm on the girls side on this one.