It's a hypothetical case, but one priests frequently face in an American culture transformed by the Sexual Revolution. 

On the other side of the desk is a couple seeking marriage-preparation sessions before a church wedding. At least one of these young people is from a parish family and, thus, has been receiving Holy Communion. Neither has been to Confession in years. 

 The pastor has every reason to suspect that, like millions of Americans, this couple is already "shacking up."

A Catholic priest knows that the catechism teaches that sex between an unmarried man and an unmarried woman is "gravely contrary to the dignity of persons and of human sexuality which is naturally ordered to the good of spouses." He knows that it teaches that anyone "conscious of a grave sin must receive the sacrament of Reconciliation before coming to Communion."

So a painful question looms over these encounters: Don't ask, don't tell?

"What I have heard priests say is that if people come to us to get married, then we don't feel like we can refuse them," said Scott Stanley, co-director of the Center for Marital and Family Studies at the University of Denver. The thinking seems to be that "getting these people married will solve the big problem that, from the church's point of view, exists in their lives."

But when it comes to addressing doctrinal issues linked to cohabitation, "you get the feeling that priests just don't know what to do right now," he said.

Meanwhile, cohabitation has turned into one of the dominant forces shaping new marriages and homes, with a majority of Americans in their 30s saying they have lived with someone outside of marriage. And new studies, argued Stanley, show that the practice of cohabitation has for many become "de-linked from marriage" altogether, with more and more people moving from one cohabitation relationship to another -- a practice with serious implications for the stability of future unions.

While most couples used to think of cohabitation as a "trial marriage," there is evidence this is no longer true. The key is that living together before marriage has become "fundamentally ambiguous" as a sign of faithfulness and commitment. Instead, it's a practice "with no implications about the odds of marrying," one that Stanley has called "CohabiDating."

This reality, for clergy, raises big questions as they deal with couples preparing for marriage, especially in churches that view marriage as a sacrament.

At the end of the 20th Century, the U.S. Catholic bishops were already circulating materials noting that nearly half of all couples seeking Catholic marriage-preparation sessions were already cohabitating. A set of 1988 guidelines, entitled "Faithful to Each Other Forever," warned priests to avoid two extremes: "(1) Immediately confronting the couple and condemning their behavior and (2) Ignoring the cohabitation aspect of their relationship."

Ever since, priests have been asked to view marriage-preparation sessions as chances to welcome couples back into the life of the church. However, they are also supposed to communicate that sex outside of marriage is grave sin.

Thus, some Catholic dioceses -- but not all -- urge those who are cohabiting people to live separately and cease premarital sex while preparing for the Sacrament of Marriage. The catechism also teaches that it is "appropriate for the bride and groom to prepare themselves for the celebration of their marriage by receiving the sacrament of penance."

While cohabitation remains a scandal for many traditional believers, guidelines from the U.S. bishops note that more and more young people -- along with their parents -- will be scandalized by clergy attempts to require cohabiting couples to repent and quit "living in sin" before a church wedding.

Rather than hiding from evolving trends linked to cohabitation, said Stanley, clergy must address these issues more often so that young people will know the faith's teachings long before couples face complications of this kind.

Clergy must be willing, he said, to "stand up and tell people that there is good evidence and good research indicating that God had your best intentions in mind when he came up with this whole marriage thing and set some standards for how you prepare for it. …

"Cohabitation isn't teaching people how to be committed to each other for a lifetime. Instead, it's teaching them how to pack up and move on."