This man and this woman are not, for each other, replaceable and substitutable individuals but are rather irreplaceable and non-substitutable persons.
Through their own free and self-determining choices, they have given to themselves and to one another a new kind of identity, and nothing they subsequently do can change this identity. They simply cannot unspouse themselves. They cannot make themselves to be ex-husbands and ex-wives any more than I can make myself to be an ex-father to the children whom I have begotten. I may be a bad father, a terrible father, but I am still my children's father. I may be a bad husband, a terrible husband, but I am still my wife's husband, and she is my wife.