NEW YORK-In a Hollywood museum specializing in erotica, there lies a grainy tape of a woman having sex with a man on a couch. The museum says it’s widely believed, though denied by her estate, that the woman is Marilyn Monroe, circa 1948.

Fast-forward some 60 years, past Rob Lowe and Pamela Anderson and Paris Hilton. The latest celebrity sex tape contains 14 minutes of seriously hard-core action between actor Colin Farrell and a former Playboy Playmate, punctuated by dialogue like: “Where’s the zoom on this?”

But like his predecessors in the genre, Farrell’s career is not likely to be harmed at all. In fact, it could even get bigger. It seems what we expect from our celebrities is radically different from what we expect from, say, our politicians. Or ourselves.

“The public is very forgiving,” says Kate White, editor-in-chief of Cosmpolitan. “And very intrigued. It’s not like, `Oh no Colin, not you!”’

“It makes him more interesting,” she says. “It gets his name out there. Now, if you or I had our own sex tape released … we’d be in deep doo-doo.”

It wasn’t always thus. In the old days, Hollywood studios worked hard to suppress behaviour they didn’t like. Celebrities under contract had to sign morals clauses, says Jonathan Kuntz, a film professor at UCLA who specializes in Hollywood history.

Kuntz is not familiar with the sex film thought by Hollywood’s Erotic Museum to be that of Monroe. But he says he wouldn’t be surprised. “Back then, she was a starlet just trying to make her way,” he says.

But the celebrity sex tape really emerged decades later, along with cheap and accessible home video equipment. In the late 1980s, at the Democratic National Convention in Atlanta, Rob Lowe became infamous for a videotaped encounter with two women, one a minor, in his hotel room. But the Brat Pack actor climbed back; he later got the plum role of presidential aide Sam Seaborn in The West Wing.

A decade later, Baywatch star Pamela Anderson and her rocker husband Tommy Lee were subjects of a sex tape that was stolen from their home and distributed on the Internet. It became an instant classic. Anderson, who was later involved in yet another sex video with another man, went on to launch her own TV series, V.I.P., which ran for four years. She now stars in the Fox sitcom Stacked.

All this is but a prelude, of course, to the story of Paris Hilton.

In late 2003, Hilton was a mere hotel heiress and socialite favoured by the paparazzi for her blond looks, skimpy outfits, nightclub escapades and celebrity friends. Her sex tape, made with then-boyfriend Rick Salomon in eerie night-vision green, surfaced just before the start of her reality TV series, The Simple Life.

It helped propel her to superstardom: a fragrance; a nightclub chain; a CD; memoirs; countless mentions in late-night monologues. In Yahoo’s compilation of the top web searches of 2005, “Paris Hilton” was the seventh most-searched term, in all categories, globally. And her photograph was the second-most searched image.

“If you’re a U.S. senator and your sex tape is leaked, it could end your career,” says Robert Thompson, professor of popular culture at Syracuse University. “If you’re Paris Hilton, it makes your career.” Why? Partly because the tape fits exactly what Hilton was trying to sell in the first place: something sexy and provocative. (It might be different if, say, Tom Hanks were to be discovered in such a tape.)

Also, celebrities are increasingly breaking the boundaries of sexual explicitness anyway. In movies and on cable, full-frontal nudity is no longer that rare. And consider 2003’s The Brown Bunny, which ended with a prolonged oral sex scene between writer-director Vincent Gallo and co-star Chloë Sevigny. What’s so different about a sex tape?

“It becomes just another media in which these celebrities work,” Thompson says.

And so, it would appear that Colin Farrell has little to worry about, despite the shockingly racy nature of his video romp with ex-girlfriend Nicole Narain, who is now trying to sell it (a trial date was set last week in his lawsuit against her.) The Irish actor, star of Oliver Stone’s Alexander and the upcoming Miami Vice, is currently in theatres with The New World, the revered Terrence Malick’s film about American Indians and English settlers in the 17th century. The film has received mixed reviews.

“It’s possible that more people will see him in the sex tape than in the movie with Pocahantas,” says White. “And maybe that’s a good thing.”

Well, with one caveat. He’d better look good in the video. Because one thing that definitely will hurt a celebrity’s career is looking bad, showing paunch or bags around the eyes or greasy hair.

“Now that,” Thompson says, “would be a scandal.”