Forgetting Sarah Marshall, this week's Hollywood rom-com, boasts the performance that everyone's talking about. “Captivating,” says the LA Times. “Brave,” says the Village Voice. “A Hoot,” says the Los Angeles Daily News. However, what's most remarkable about this turn is that it comes not from a seasoned thesp or a white-hot newcomer, but from the vaguely inanimate genitals of the leading man, Jason Segel.

Specifically, in the movie's one memorable moment, amiable schlub Segel, freshly emerged from the shower, is dumped by his titular girlfriend (Kristen Bell). Here repeated and unflinching shots of the naked Segel, hands covering his mouth in shock, groin utterly exposed, show us that his penis, ultimately, is the star of this set-piece and the one that gets the most laughs.

This phallocentric showstopper is a revolutionary break from Hollywood tradition. Mainstream studio movies have always complied with Freud's dictum that the sight of genitals, especially male, though exciting, “can never be considered beautiful”. For Hollywood, in fact, the idea that leading men possessed anything more troubling between their legs than a smooth Action-Man bump was incomprehensible to an audience fed on beefcakes such as Kirk Douglas in Spartacus, or Burt Lancaster in From Here to Eternity - both real men, yet both carefully dressed in strangely asexual, non-revealing trunks.

Despite the collapse of the studio system and changing moral climates, this aversion towards depicting male nudity has been resilient. It was occasionally challenged with brief glimpses and stolen shots, such as Oliver Reed and Alan Bates flipping and flapping through the nude wrestling scene in Women in Love (1969), or Richard Gere (below) giving his audience a momentary groin shot in American Gigolo (1980). And yet still mainstream male nudity is either confined to the microsecond glimpse factor (see Viggo Mortensen's nude fight scene in last year's Eastern Promises) or else to a select few so-called “swordsmen”, who, one suspects, deep down, are quite enamoured by their own, yes, talents. The bulky, hulking Harvey Keitel has never been one to refuse a money shot, boldly revealing himself for both The Piano and Bad Lieutenant.

Similarly, Ewan McGregor, for a while, seemed to have a penis clause written into every contract, from The Pillow Book to Trainspotting to Young Adam - culminating in the scene from Velvet Goldmine where, as ersatz glam-rocker Curt Wild, he exposes himself live on stage, swinging his member frenetically, like a mini microphone on a rope.

What is unusual, though, about Segel's nudity in Forgetting Sarah Marshall, is the banality of the context. This is not sexualised imagery here. It is a man talking to his girlfriend about their relationship. The camera is planted firmly, it seems, at waist height. The penis here, without doubt, is the focus of the scene. It is a groundbreaking move, from Segel and the film itself, for it frees the penis from heavyweight symbolism and primal connotations, and it makes it instead a thing of fun, and of normative male identity. The real question becomes one of urgency and speculation for the whole industry - who's going to follow Segel's lead? Who will take up the, er, baton? Cruise? Pitt? Depp? Come on guys, you know you want to...

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