I am on friendly terms with all of my exes. Some were seriously mono relationships, two were open.
I know people who think thats odd, my husband was bothered by it when we started dating. But I just don't stop caring about someone because I establish we aren't a good romantic duo any more.
I consider it a warning sign if someone can't get along with any exes. It could be that they have very few and those were circumstantially not worth remaining on friendly terms, could be that their exes didn't want to be friends. But it also could be that they are just bad at breakups/ blaming others instead of finding where they went wrong and using endings as options for introspection and self improvement.
I have gotten MUCH more picky about who I will consider a potential date since becoming actively poly. If someone isnt HIGHLY motivated to self improvement, I don't need the added responsibility. I suspect this will result in far fewer breakups and will also help continue my ability to remain friends with any exes.
"Love As Thou Wilt"