I am friends with many of my exes. Have gone back and tried to 're-date' several. Kinda how I got into this poly thing, was staying friends with my last ex. I did date his best friend, so it would have been weird not to talk.
My high school bf (the longest term one) looked me up after his wife passed. He was really looking for a new mom for his kids though; and all the reasons I never married him the first time were still existant, so I passed on that. I am not friends with either of my two long-term girlfriends, though when I was with the second, I made attempts at being friends with the first. We did work in the same (huge) organization. Lotsa drama in my twenties.
I had a really wise therapist advise me to take 40 days and 40 nights complete break before changing a relationship (no contact whatsoever). It's usually been one-sided (mine); but they have cooperated nicely. When I don't do that, it usually goes badly for me. That therapist was friends with everyone, and always wanted to know her lovers' entire histories, with details.
I figure if I like and love you well enough to do you; I'm not going to stop liking you /loving you just because we don't want to have sex anymore. When I had the breakup conversation with First boyfriend, we went round for an hour or so. But I had talked to him early on, and he had said that he had never stayed in contact with people he broke up with. So I brought that up. And I started crying at that point, and saying how much I liked him, and if I had to go away thinking we could never hang out again, I'd be really broken up about that. And that's when he started crying (and he got mad, and said 'there, are you satisfied? now I'm crying.')(and I was, a little). But I figured that would mean we would come round to friendship. And it had a LOT to do with the magick of Current boyfriend. He is naturally gregarious and inclusive. In his wildest dreams, it never occurred to him that three of us would not be good friends again. My favorite picture of us is the day that we were first all together after the breakup. We had lunch at a local pub and it was just a very sweet reunion. (I also found out that day that First bf's new girl had dumped him)
So, maybe it's not poly per se; but in me, perhaps it is.