What is marriage? Look at this ancient institution from an impartial distance, with all Hollywood movie and romance novel illusions aside.

The institution was invented by society thousands of years ago for one purpose: to insure the survival and propogation of the species, and in doing so, society itself. Why does society have to impose and police this "contract" between 2 people? Because we as individuals are weak and vulnerable to temptation, greed, and laziness, and as such could never self-police the extreme demands and sacrifices that marriage demands of each of us, especially during child-rearing years.

In short, without society-enforced marriage, very few pairs have the mental perseverance or physical stamina to remain together long enough to properly raise children into emotionally stable adults, capable of marriage themselves, and repeating the process into the future, thereby insuring the survival of society itself.

The "logical" answer to the question "what can be considered a failed marriage?" that falls out of this argument (I can feel the hundreds of cursors hovering over the "groan" button) is this:

A failed marriage in this sense can be view as one that did not produce children. That is not to say the relationship has necessarily failed, and certainly not all couples are biologically able to conceive. No one is to blame if a marriage did not produce children, whether by choice or circumstance. It is rather like saying a promising athlete failed to win a medal due to ill-fitting shoes, or bad weather. In either case the medal wasn't achieved, and that was the goal of the event. The athlete did nothing wrong per-say.