Adoptive and birth parents do not always meet even when a reunion occurs. However, it can be beneficial for both sets of parents to meet each other. Sometimes adoptive and birth parents from closed adoptions meet and form a warm relationship. It seems that is not the usual outcome. Although I do know of a few instances in which that is the case, it is more likely that both sets of parents do not meet or they meet yet there is a great deal of tension between them and relations are strained.
There are a variety of reasons why the adoptive and birth parents of a child may not become friendly towards each other. While I do not believe that it is necessary for them to become friends, to be cordial and civil to each other may be necessary if there are grandchildren in the picture.
About a month ago I received an email from a very anxious birth mom who was about to meet the adoptive parents at her son's insistence. She was quite reluctant to meet her son's adoptive parents, yet she felt that since the occasion was a religious ceremony for a grandchild she wanted to be there. One of the most difficult factors for her was that she knew that the adoptive mom had been abusive to her son as a child.
Most birth mothers do not have this issue to deal with. It does complicate the situation. When an adoption begins as an open adoption, but is shut by the adoptive parents is another instance that will make becoming friends difficult. However, particularly in closed adoptions, there is often no reason why both sets of parents cannot become friendly.
The chief advantage of both sets of parents at least being cordial to one another is that it may take the child (adult or not)out of the middle. The balancing act with two sets of parents is often trying for adoptees. Trying to keep everyone happy and not offend anyone can be a great deal of work.
Parents can become a issue if they are always requiring that the adoptee choose between them. For an adoptee to not need to worry about both sets of parents getting along can be a huge relief. Although it may not be easy, for the adoptee's sake, and everyone's, it is best if there can be peace among both sets of parents.