The Church holds Catholics to a higher standard. In Luke 12:48 Jesus says: “Everyone to whom much has been given, of him will much be required…” No entity on earth holds marriage in as high esteem as does the Catholic Church. Protestants don’t even acknowledge it as a sacrament.
In the case you mention, the atheist’s marriage was valid, but not sacramental. The atheist was sincere in his ignorance, but was not capable of entering into a sacramental marriage because he as yet did not have the grace of faith nor the grace of baptism.
Let’s look at what a Catholic is doing who chooses to marry outside the Catholic Church. Certainly, such a person may do this in ignorance. But the objective reality is what we must consider here. Such a person is throwing away the most precious gift one can have, i.e., one’s gift of faith.
We believe that Jesus founded a Church and gave authority to that Church in the person of St. Peter and to his successors. “Whatever you bind on earth will be bound in heaven, and whatever you loose on earth will be loosed in heaven.” (See Mt. 6:13-20) To step out of the Church and attempt marriage is to reject that authority. It is a step away from what one has committed oneself to by being a Catholic. Either the Catholic Church is THE Church founded by Christ with the authority He gave it, or it isn’t! Either marriage in the Catholic Church is a sacramental sign of the marriage bond that Jesus has with His people and all else that the Church teaches about it—or it isn’t.
There is no lack of logic or justic here on the Churchs’ part. There is on the part of the Catholic who chooses to act in contradiction to his commitment as a Catholic!
Fr. Vincent Serpa, O.P.