Stephen J. Heaney is an associate professor of philosophy at the University of St. Thomas, and while he does not speak for it, he is happy to share his personal views as a Catholic and as a resident of Minnesota:
John Nienstedt, who leads the Archdiocese of St. Paul and Minneapolis, has been subjected to much disrespectful criticism -- from members of his flock, and even from a Lutheran bishop -- regarding his lead role supporting the marriage amendment. He stands accused of forcing his religion on everyone, and of forcing the consciences of his fellow Catholics. Though not unexpected, the charges are unfair.
What is the dispute about? One side holds that marriage is a vow of a man and a woman before the community to engage in a project that is greater than the couple, and that the community should hold them to it. This has been the universal view of marriage since time immemorial. The other side thinks that marriage (or civil union) is an expression of the desires of two people (their gender doesn't matter), and that the community must support them whatever they choose -- to have sex (and perhaps children) within marriage or not. To accept one definition, one must reject the other.
Due to the encroachments of the revisionist view of marriage into law, typically by judicial fiat, the institution is in some danger. Thus, those who wish to protect that time-honored institution seek to define it in the state Constitution.
This essentially is their argument. There is only one reason the community is interested in the friendships and sexual arrangements of human beings: When a man and a woman are united sexually, the natural (and frequent) result is children. Children on the whole do best, by any measure, when they are united with their biological parents, who are themselves united before the community to each other. If sex did not lead to children, no one would ever have thought up the institution of marriage. It would be a strange, intrusive insinuation of the community into the lives of its citizens. On the other hand, this union of man and woman, and the resulting children, are the cornerstone of every society that has ever existed. -- StarTribune