Many thanks, Renee, but I am puzzled by your reply.
Suppose we have
Three parents, Parent 1, Parent 2 and Parent 3 and Parent 2's first marriage was to Parent 1 and second marriage to Parent 3.
From the first marriage a child was born.
At the moment, as far as I can tell, the line of descent to the child will emerge from either Parent 1 or Parent 2 and not from the line showing a marriage between them.
If, for example, it emerges from Parent 2 it would not be possible to tell whether the child was a child of Parent 1 or 3.
And if Parent 1 also had a second marriage and the line of descent emreged from him/her it would not be possible to tell whether any children were the children of Parent 2 or Parent 1's second partner.
Having the line of descent joined to the line of the relevant marriage or relationship, makes this clear. And this is what I would like to be able to achieve.