Gaudium et Spes says the following about marriage:
Thus a man and a woman, who by their compact of conjugal love “are no longer two, but one flesh” (Matt. 19:ff), render mutual help and service to each other through an intimate union of their persons and of their actions. Through this union they experience the meaning of their oneness and attain to it with growing perfection day by day. As a mutual gift of two persons, this intimate union and the good of the children impose total fidelity on the spouses and argue for an unbreakable oneness between them.(2)
Now in the case of civil divorce, or spousal abandonment, it is clear that there is no longer any real marriage in the above sense expressed in Vatican II. There is no rendering of mutual help, no growing in oneness, nothing at all. The only thing that remains is the juridical bond that prevents remarriage to another person. To pretend that someone in this situation is identical to someone in a real, ongoing marriage having the above characteristics is just plain silly. And I don’t think the Catholic Church is silly.