You said something very important. A relationship should be build on trust. Once ex in most cases the trust is gone if it ever been there.

Indeed no one likes to be the ex but once adult we are exes in many ways and the word 'ex' part of 'exit' says enough. It is the end.

I wonder how it is for the new partner to be confronted with the ex. Even if the relationship is good a wedding should be about the new couple not exes sharing good memories, a past the new partner most likely was no part of.