<<<which is the primary relationship to BF while the children are grown, it is the spouse or the grown child?>>>
Once a child is an adult the responsibility to "be there" for the child, and have a relationship with the child ends. The vast majority of parents will continue to have a relationship with their grown kids and will want to help them from time to time, but the actual obligation to do so has ended.
My mother doesn't reach out to communicate with me unless she wants something. It sucks, but we're both adults and that is her prerogative. There is, of course, a consequence for her lack of interest in that I may not be there when she wants something or if she decides that she now wants more of a relationship with me (or my sister...we're both equally ignored by mom).
<<<Does SM have an obligation to even speak to the child ? The BM?>>>
No and no.