In the Bible. Which the Church has received the authority to interpret (it being her book and all).
Human conscience is where the concept of marriage for life was first recognized. There is no such thing as the "moral death" of a marriage, any more than there could be the moral death of someone's human nature. The practice of people degraded to conditions of slavery can hardly be considered normative or consistent with human nature, I'm thinking. So your example is meaningless.