VIEWER discretion is strongly advised.

Here’s the naked truth: Male genitalia is getting unprecedented screen time at the multiplex these days -- in mainstream popcorn fare and broad comedies -- thanks in large part to comic mogul Judd Apatow (and his band of merry collaborators), who has pledged to shake Americans from their squeamishness about male anatomy in movies.

Exhibit A: the Apatow-produced R-rated heartbreak comedy “Forgetting Sarah Marshall,” which hits theaters Friday.

Judging by reactions at its South by Southwest Film Festival premiere last month, “Sarah Marshall’s” breakout performance isn’t delivered by protagonist Jason Segel (of “How I Met Your Mother” and “Freaks and Geeks” fame). The movie’s most captivating screen presence -- and its already much buzzed-about primary talking point -- is what a Harlequin romance novel might refer to as Segel’s “manhood.”

The 28-year-old comedian is presented in his full-frontal glory over the course of a cringe-worthy breakup scene that involves Segel naked, dripping wet and sobbing like a 6-foot-4 baby. No clever “Austin Powers"-style genital obfuscation with props. No artful lighting to preserve any mystery about Segel’s man junk. And for emphasis, the camera cuts away from close-ups of his teary visage (he’s being dumped by his girlfriend, Kristin Bell, as the titular Sarah at the time) to wider, phallus-inclusive shots three times.

At SXSW (as the Austin, Texas, film fest is known), every time Segel was shown au naturel, the totality of his package nearly brought the house down with laughter.

Segel also wrote the film’s script, basing the scene on a real-life naked breakup of his own. Still, the actor-writer contemplated inequalities between male and female film nudity before putting his family jewels on display.

“When a woman does nudity in a movie, men immediately switch into a sexual mode,” Segel said. “For women, from what I understand, it’s not like that. They see a naked, out-of-shape man crying and it’s funny -- something weird, disturbing and disgusting we can all laugh at.”

To hear him tell it, Apatow -- the comedy rainmaker behind “The 40-Year-Old Virgin” who has shown a Midas touch producing such jocular male-skewing comedies as “Knocked Up,” “Talladega Nights: The Ballad of Ricky Bobby” and “Anchorman: The Legend of Ron Burgundy” -- has made it a sort of personal mission to up the on-screen male member quotient.

Call it a crusade to break down one of moviedom’s last taboos. In the 2007 faux musical biopic he produced, “Walk Hard: The Dewey Cox Story,” Apatow positioned a penis behind the film’s star John C. Reilly’s head during an orgy scene. The upshot: Some 20 audience members reportedly stormed out of a test screening in disgust. As well, the closing credits for another film Apatow produced, the teen comedy “Superbad,” feature a hilarious illustrated montage of male sexual organs.

“America fears the penis, and that’s something I’m going to help them get over,” Apatow is quoted as having said in a World Entertainment News Service story in December. “I’m gonna get a penis in every movie I do from now on. . . . It really makes me laugh in this day and age, with how psychotic our world is, that anyone is troubled by seeing any part of the human body.”

Likewise, the raunchy stoner comedy “Harold & Kumar Escape From Guantanamo Bay” (due in theaters April 25) is doing its part to counter phallic phobias. In the film’s hot tub party scene, a naked supporting character’s pubic hair prompts comparison to Osama bin Laden’s beard.

In letting it all hang out, Segel joins a select boys club of big-name stars who have gone memorably bottomless. Among them: Tom Cruise in “All the Right Moves” (1983), Bruce Willis in 1994’s “Color of Night,” Daniel Day-Lewis in “Stars and Bars” (1988), Richard Gere (in 1980 for “American Gigolo” and again in 1983’s “Breathless”) and Harvey Keitel (for 1992’s “Bad Lieutenant” and 1993’s “The Piano”).

Ewan MacGregor deserves special notice for his appearances in four films that stand in contrast to his iconic turns as Jedi master Obi Wan Kenobi in the “Star Wars” prequels. The Scottish actor flashed his light-saber for the independent films “Trainspotting” (1996), “The Pillow Book” (1996), “Young Adam” (2003) and “Velvet Goldmine” (1998).

“If doing full frontal gets me into that club, I’m happy to do it,” Segel said.

Ironically, the spike in male movie nudity comes at a time when actresses are more and more reluctant to take it off on film for fear of being immortalized in the buff on websites dedicated to nudie film stills and screen grabs (much like the website the characters try to get off the ground in “Knocked Up”). Elisha Cuthbert and Eliza Dushku have publicly stated that they won’t do nude scenes, as did Lindsay Lohan in 2005; this year, she posed in the buff for a New York magazine layout.

“It’s more of a concern for actresses now than it has been in times past,” said Mike McPadden, editorial director of mrskin.com, a site dedicated to “celebrating nudity in film & TV.” “Nudity is instant and permanent. We get it right away, and it lives forever. So when it does happen, it’s a big deal.”

Below-the-belt word-of-mouth has accounted for much of the pre-release awareness for “Sarah Marshall.” But not everyone associated with the film is overjoyed by the film’s conspicuous display of beefcake. British comedian Russell Brand, who portrays the preening rock star for whom Sarah cuckolds Segel’s character, admitted to a certain actor’s remorse upon seeing the movie for the first time.

“If I had known there was going to be so much male [genitalia],” Brand said at a party for Facebook.com during SXSW, “I probably never would have agreed to be in it!”

chris.lee@latimes.com