To some young Catholics, the sacramental bond of marriage seems scarcely distinguishable from a civil bond, or even a purely informal and open-ended arrangement to live with another person. Hence we have an alarming decrease in the number of Catholic marriages in the United States together with an increase in cohabitation, in which the Christ-like mutual self-giving of spouses, sealed by a public promise to live out the demands of an indissoluble lifelong commitment, is simply absent. In such circumstances, children are denied the secure environment that they need in order truly to flourish as human beings, and society is denied the stable building blocks which it requires if the cohesion and moral focus of the community are to be maintained.
This is the beauty of Catholic teaching. People often believe Catholic teaching is nothing but a set of prohibitions. But, here, Benedict stresses what the Church is positively in favor of: Marriage is more than a civil union, more than a contract. Marriage is the lifelong commitment of mutual self-giving. You can't have that in cohabitation or in a union that is understood to be temporary.