BALTIMORE -- Despite their battles over same-sex marriage, the nation's Catholic bishops say their greatest challenge is convincing faithful Catholics that only a vowed, lifelong commitment between a man and a woman can be called marriage.

"When questions such as, 'If marriage is so good, why can't everyone have it?' come not only from the outside but from those within our own flocks, it is a reminder of our important task," Archbishop Joseph Kurtz of Louisville, Ky., told the bishops assembled in Baltimore this week for their semiannual meeting. His Committee for the Defense of Marriage will launch a series of projects to teach Catholics why the church believes marriage is intended to be lifelong, between one woman and one man and open to children.

The church hasn't explained that well to those in the pews, he said. Focus groups found that self-identified practicing Catholics were unaware of church teaching that men and women are equal but different, and that marriage is intended to unite those differences in a complete whole.

"It was not so much that they reacted against those concepts. Rather, the significance of sexual differences -- that God created them male and female -- often seemed absent in their reflections, completely missing," Archbishop Kurtz said. "To provide an effective message on this theme will require taking our young adults seriously where they are and initiating a conversation with them, presenting the message well, and more than once."

He previewed a film yesterday that his committee will release next year, intended primarily for Catholics in their 20s. It opened with scenes of nature as biblical words about marriage played across the screen. "In the beginning, the Creator made them male and female," one said.

Then it focused on a young couple talking about what made their marriage different from any other relationship. It's intended as a conversation starter, to be used with other teaching materials.

After the film, Bishop Richard Malone of Portland, Maine, thanked the bishops for their personal and financial support during a recent successful campaign against the legalization of same-sex marriage in his state. Advocates of gay marriage "are counting on the young people to buy into all the distorted thinking around civil rights and equality. We have to do all we can with this type of ... educational material," he said.

In a pastoral letter adopted at the meeting, the bishops say same-sex marriage isn't a civil right because humanity has always recognized that marriage is between a man and a woman. They believe that redefining marriage threatens the foundation of society.

"Today, advocacy for the legal recognition of various same-sex relationships is often equated with non-discrimination, fairness, equality, and civil rights. However, it is not unjust to oppose legal recognition of same-sex unions, because marriage and same-sex unions are essentially different realities," the letter said.

Contacted later yesterday about the bishops' stance, Francis DeBernardo, executive director of New Ways Ministry, a ministry to gay Catholics that is not recognized by the church, pointed out that the bishops don't oppose legal divorce even though that violates the church's understanding of what God intended marriage to be.

"The Catholic Church might believe that marriage is only between a man and a woman, but it's a form of discrimination when it doesn't allow secular society to define marriage the way that it wants to," he said.

"If the bishops want to strengthen marriage, they should support marriage across the board. Same-sex marriages will strengthen heterosexual marriages because it will be a support to committed love."

He said he doesn't believe there is a universal definition of marriage.

"Marriage today is not the same institution that it was 50 years ago or 1,000 years ago. It has always changed based on society's understanding of relationships. And we have a new understanding that homosexual relationships are normal and natural and holy," he said.

When Archbishop Kurtz presented the letter, he misread his text to say that marriage was "between two adult couples."

He caught his mistake, but said it illustrated his point about trying to redefine marriage.

"That's a real slip of the tongue and it's where we may be going if we don't do some work," he said.

Washington correspondent Daniel Malloy writes the "Pittsburgh On The Potomac" blog exclusively at PG+, a members-only web site of the Pittsburgh Post-Gazette. Our introduction to PG+ gives you all the details.