I think many polys have a different attitude toward "ex-es" than monogamists. This is understandable. A poly’s emotional involvement is not with only one person. If you break up with someone, you may have one or several partners still remaining.
In addition, polys don't need to end one relationship before beginning another one. I don't have to "dump" A before dating B. I don't have to wait for C to dump D before asking C (or D!) out.
So perhaps we don't feel the same pressure to "trash" an ex that some monogamists feel (though we may still trash them!)
It is probably easier then for us to retain some sort of friendship with our ex-es (unlike monogamists). We do not have to dump a new love to get back with an old one. We can acknowledge, respect and learn from our ex-es.
All this affects our attitudes towards our past loves. I wouldn't at all mind staying in contact with people I have dated, known, or been close to.
On the whole, I've been lucky. It’s true I’ve been dumped or rejected by monogamists. Some of them wanted nothing to do with me because I was poly. But some loves who married and became monogamous are still my friends.
Then there are the polys I've dated -- some still talk to me, some don't. I wouldn't mind still knowing them.
I guess it depends how the relationship ended. Some monogamists cut you off as if you were a contagious disease. The ones I respect are the ones that continue the friendship – even accepting that I still want them -- even though they've realized they personally want monogamy.
On the other hand, I've been involved with polys who cut me off. At least one was a person who, in my opinion, was showing signs of "going monogamous"...
I've also sometimes come to specific crossroads, with both polys and with monogamists, where we just weren't interested in each other any more. I guess I've been dumped by one person who was definitely monogamous, by another person who had been militantly poly but became disillusioned when the polys she knew didn't live up to her standards, and by a definite poly when we each labeled each other "unstable" (hmm...I wonder which one of us was "projecting"!).
Things don't always "work out" -- but I think I've learned something from all of my past involvements. On the whole, I still respect them and care about them; even though in some cases we hurt one another.