I can even take that concept another step.
My current has remained good friends with his ex,for the same reason others have stated: they were very good friends before and during their long relationship! When I relocated here to continue my relationship with my partner (now 7 years), his ex has become one of my best friends! ANd I could not have imagined such a gift.
As for my own ex's, we (2 of them) have also remained friends. Life is too short to harbor negative energy..each man was a learning experience,creating the man i am today and the relationship i know cherish!