(UNDATED) So comedian David Letterman, 62, jokes that Gov. Sarah Palin’s teenage daughter got “knocked up” by 33-year-old Alex Rodriguez during the seventh inning of a Yankees game. At the time, he didn’t specify which daughter, but the only one at the game is just 14.

Not to be outdone, Calvin Klein unveiled a soft-porn billboard a few blocks south of Letterman’s studio in Times Square. Half-naked teens cavort on a couch in a manage a trois, while a forth lies dreamily on the floor below.

Letterman’s joke and Calvin Klein’s ad send the same message: teenage girls are easy prey.

Heard any comments from Catholic bishops? You probably won’t.

Approximately 80 U.S. bishops criticized President Obama’s invitation to the University of Notre Dame, complaining about his stance on abortion, but are any bishops complaining about porn-ads?

Anyone? Anyone? I didn’t think so.

The U.S. Conference of Catholic Bishops routinely puts out elegant statements against embryonic stem cell research, but what about Letterman? Where’s the purple and red tide in defense of teenage girls?

If you think it’s because abortion and stem cells are “religious issues,” while teenage abuse is not, think again. This would be an opportune time for some bishop — any bishop — to chime in and say that sexual exploitation of teenage girls in the media gives energy to perversions elsewhere, including in the church.

The comments about Palin’s daughter are outrageous. You would think (or hope) the bishops would have their cassocks in a knot. Letterman actually suggested sexual activity between a teenager and an adult male. Take a look at http://www.bishopaccountability.org, and you’ll see stories — real, verified stories, not sleazy jokes by some aging comic — that are much the same. About priests. And bishops.

Letterman’s defensive statements sound like they were written by diocesan attorneys. First, Letterman said he thought it was her 18-year-old daughter, who already had a baby out of wedlock, who was at the game. He checked, he said, to be sure she was of legal age. Now he says he had no idea anybody was at Yankee Stadium besides Palin and former Mayor Rudy Giuliani.

Then there’s Letterman’s sleaze-brain comment about Palin’s “slutty flight attendant look.”

Will Letterman lose his job? Here’s a statement from CBS president Leslie Moonves: “There has been much discussion of the effect language like this has on our young people, particularly young women…” The problem is, that’s what Moonves said — two years ago — when he fired CBS radio host Don Imus for calling the Rutgers’ women’s basketball team “nappy-headed hos”.

This time around, CBS shows no signs of letting Letterman go. Their official statement this time? “No comment.”

In Los Angeles, talk show host John Ziegler (who calls himself a “recovering Catholic”) has begun a Fire David Letterman campaign. A few advertisers have withdrawn their sponsorship. Some folks have demonstrated outside Letterman’s studios.

Yet it’s not only about Letterman and Calvin Klein and bishops who prefer to look the other way at these events and so much more. It’s about defending life.

Letterman goes home to his 108-acre estate and counts his money while poor teenagers are exploited by dirty old men. That’s humor? Tell that to the homeless teenage girl who just left her firstborn at a firehouse.

Calvin Klein runs its perverted ads and the city rumbles on below. It doesn’t matter? Tell that to the lonely daughter of a single mother who just shoplifted a pair of Calvin Klein jeans in hopes that she, too, will get some attention.

The bishops have more important things to think about? Tell that to the girl who once got propositioned by her pastor.

It is about life. Think about it.

(Phyllis Zagano is senior research associate-in-residence at Hofstra University and author of several books in Catholic Studies.)