It's easy to take a shot at the Catholic Church for weighing in on the issue of gay marriage.

There is a very real prospect this summer that the state's highest court will declare that the traditional definition of marriage - that is, between a man and a woman - is unconstitutional. It's a prospect that has alarmed Boston Bishop Richard G. Lennon as well as the bishops of Worcester, Springfield and Fall River. Last weekend's Sunday Masses featured a letter from the four urging congregants to lobby for a constitutional amendment that would reverse the court's feared decision.

Some walked out while the letter was being read. Most stayed and listened. But it's a safe bet that few will be doing what the bishops asked.

Why?

The most obvious answer is that the wind is out of the church's moral sails; clergy today have as much credibility around matters of sexual morality as Ted Kennedy did during the Clarence Thomas/Anita Hill hearings. That, of course, is due to the sex scandals involving priests and children, an evil that the hierarchy covered up and ignored for decades. The attack on gay marriage reminds some of the words in Matthew: ``How canst thou say to thy brother, Let me pull out the mote out of thine eye; and, behold, a beam is in thine own eye?''

Yet it's not only the church's lack of credibility that's at issue. As with other matters, such as contraception and divorce, the beliefs of those who call themselves Catholics (and that's about 65 percent of the 3.2 million who live in the Boston archdiocese) are increasingly at odds with the positions of the church.

The bishops are clearly worried. Although they are careful to say that gays ``deserve our respect, compassion, understanding and defense against prejudice,'' they at the same time argue that gay relationships are second class. ``No same-sex union can realize the unique and full potential which the marital relationship expresses,'' say the bishops. One wonders how in the world they can possibly know that to be the case. And most gays would say that the bishops are flat-out wrong; some who were once closeted and married clearly find their uncloseted, gay relationships to be far superior.

But it isn't just the allegedly inferior quality of gay relationships that bothers the bishops. More critically, they say, if gays can marry, then ``marriage as we know it will be irreparably harmed.''

It once was the case that the vast majority of the population would have shared their sentiments. Thirty years ago, homosexuality was hardly ever spoken of in public. Ten years ago, the prospect of gay marriage was virtually unthinkable; even gay advocacy groups shied away from raising the subject.

No more. Gays are now out of the closet, and they're seemingly everywhere. Gay couples with children are increasingly commonplace. Many cities, towns and private companies have recognized ``domestic partnerships.'' And gay relationships have become part of the discourse of everyday life, from groundbreaking television shows such as ``Ellen'' or the merely comical, like ``Will & Grace.''

These have all had an effect. Where once most people thought homosexual sex should be banned, the Gallup Organization says that now 60 percent nationwide think gay relations should be legal - and 50 percent think gay marriage is OK. A poll last year by the Boston Globe/WBZ-TV found Massachusetts residents to be even more supportive of gay issues.

The logic is simple. Committed heterosexual couples support each other psychologically and financially. Marriage provides a strong environment in which to raise kids. And the marital institution, with its emphasis on sexual fidelity, provides a safe haven within which relationships can become more trusting and more intimate.

It seems hard to deny the same opportunities to gays.

Indeed, contrary to the position of the bishops, gays don't undermine marriage in advocating for marriage for themselves. Rather, they are implicitly making the case for the worth of the institution itself. Far from denigrating marriage, they regard it as the ultimate expression of a close and loving relationship.

Yet the bishops are right about one thing. Marriage is in danger - not from gays and lesbians, but rather from those who trivialize the institution itself.

Young couples brag about the odious practice of ``starter'' marriages. Divorce - almost always the most traumatic event of any child's life - has become increasingly common, readily granted and often for the most slight of reasons. And even efforts to create civil unions - which many gays think a poor substitute for the real thing - have the effect of subverting the singular importance of marriage.

It is a curious thing that gay couples who love each other and want to marry stand accused of promoting the destruction of the institution of marriage. The opposite is the case: By demonstrating just how important marriage is to their lives, they are showing the rest of us how much it really matters.

Talk back to Tom Keane at tomkeane@tomkeane.com.