T here's a reason why everyone will remember the bath house fight scene in David Cronenberg's new London-set Russian mobster movie Eastern Promises. Not because it's a bloody and inventive scene, nor that it took a week to film it, but because you get to see Viggo Mortensen's willy.

Mortensen has been widely commended for realising before filming began that the scene simply wouldn't work unless he was completely naked. After all, who wears undies in the steam room? For some reason, probably having to do with America's ineradicable puritan heritage, it's always a big deal when an actor just whips it out and slaps it on the table for all the viewing public to see. It happens so infrequently that you imagine viewers squinting their eyes and asking themselves, "Did I just see what I think I just saw? Oh God, there it is again! A penis!"

And by reason of that unsettling reaction alone, I believe that there should be more male nudity in the movies - lots more. We haven't seen this much attention paid to penises since Richard Gere stood starkers in front of Paul Schrader's camera in American Gigolo nearly 30 years ago. It was as if male audiences had to pay for seeing Lauren Hutton's breasts by gazing upon the shrivelled nether regions of the officer and gentleman to be. One can almost hear Schrader muttering, like Elmer Fudd, "Oh come on, fair's fair: Lauren showed her stuff, now let's have one for the ladies."

The sight of a penis on screen never fails to get people's attention, despite the fact that said object of controversy is gazed down upon by its owners - half the planet, last time I looked - every morning at least, and is inserted into a goodly percentage of the other half of the planet last thing at night. Yet for all its mundanity, the totemic power of the phallus, rampant or dormant, to shock us has yet to subside, or, to be technical about it, detumesce. It always pokes the viewer right in the eye, so to speak, and, well, isn't that justification enough?

Onscreen nudity, both male and female, has been on a downward slide in the movies since the rise of Ronald Reagan and the religious right and the mainstreaming of the more censorious tendencies within 1970s feminism (unlikely allies, but there you go). If you grew up in the 1970s, there was nudity a-plenty in both European and American cinema. On British television before the advent of Thatcher, you could scarcely turn on the news or Jackanory without giant breasts heaving into view sooner or later. It felt like something progressive at the time - indeed, 1970s porno people often considered themselves as countercultural trailblazers - though the lopsided supremacy of female nudity was indeed a partially sexist phenomenon.

And when nudity largely disappeared, it felt like a deeply regressive step, part and parcel of whatever hellfire the Jerry Falwell wing was preaching. Just getting back at those folks is reason enough for more men to doff their jockey shorts and start waving the man-meat.