I, too, have remained friends with an ex, but he and I weren't sexually involved. We were VERY CLOSE friends, though, before dating, and remained so afterwards. He is now married and expecting his first child, and I couldn't be happier for him!
Now, my most recent ex, our relationship is definitely a little different. We are slowly but surely becoming friends. Now, this took A WHILE!! About a year and a half, roughly. Before that, I still wanted to be in a relationship with him. If one of you still wants the relationship, the friendship is basically impossible, unfortunately. But after you're over him, everything is pretty much smooth sailing.
As far as when to ask about new love interests, that depends on the personalities of the persons involved. My ex knows me, and he knows I'm nosey. So I ask whatever I want, LOL. *BUT* Only ask the question if you are PREPARED for the answer. My mother always told me not to ask a question I didn't want to know the answer to.
The fact of the matter is, if you and your ex were actually friends before or became friends during your relationship, then you will be friends afterwards. The road might be bumpy, but the friendship will remain. My ex is very special to me, so I'm willing to work (only as much as he is) to maintain a friendship with him. I would hope to be at his wedding and he at mine, yadda yadda yadda.
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