In the new movie, Ms Diaz is an orthopedic surgeon with whom men just can't help falling in love, including her old flame, Ted, played by Ben Stiller. The question is whether she will end up with Ted or one of her other suitors, and you can't get a more traditional plot than that.
But when I tell you that the two big setpieces in There's Something About Mary are the penis-in-the-zip joke and the semen-in-the-hair joke, it will become clear that Ms Diaz is not a romantic lead in the Doris Day mould.
The Farrelly brothers, whose previous credits include Dumb and Dumber, have made a movie in which Mr Stiller prepares for a big date by masturbating in a hotel bathroom and answering the door with a dollop of semen stuck to one ear. Ms Diaz, mistaking it for hair gel ... well, you get the idea.
On the day I saw a preview of the movie, President Clinton's spiritual advisers provided a detailed account of their attempts to save "a man whose soul is in mortal danger". Tough measures, said the Rev Gordon McDonald - clearly not a chap who likes dirty jokes or levity of any kind - are needed to save Mr Clinton.
Turning up at the White House two or three times a week, Mr McDonald gives the President a very hard time. "We have gone to the bottom with this man," he boasted on American television. "We have said things that are very, very confrontational."
The cynical reaction to these revelations is that they are a manoeuvre by the White House to show that Mr Clinton is truly repentant. They certainly strike me as an over-reaction to the sexual acts in the Starr report, especially in a country where audiences have already spent $100m at the box office to watch Mr Stiller masturbating.
In that sense, both the President and his Republican accusers have underestimated the degree of tolerance which is to be found beyond the Bible-belt states of the Deep South. Cinema-goers who flock to films made by the Farrelly brothers, and to other entertainments which revel in scatological humour, are hardly likely to think that Mr Clinton is going to hell in a handcart because of his affair with Monica Lewinsky.
A more alarming possibility is that the President himself believes it. He professes to, which is why he allows these busybodies through the portals of the White House.
Presidents are as entitled as anyone else to respond to Jehovah's Witnesses or Baptists who come calling, with a firm announcement - "I don't subscribe to your morality, so you can go and boil your head" - and a polite closing of the door. Mr Clinton's inability to do so is a consequence of the unresolved contradiction which got him into trouble in the first place.
It is also what makes the belated support offered to him last week by women's groups and signatories of a letter in Le Monde, including Gerard Depardieu, Emma Thompson and Carlos Fuentes, so misplaced.
Mr Clinton, they argue, should never have been required to answer intimate questions about his private life. Feminism has been placed in the dock here, as I expected it to be, for breaking down the traditional barrier between public and private life. There is something unedifying about people on the left trying to re-write the rules when the culprit is regarded as being "one of us", rather than some ranting right-wing ideologue. It cannot be said too often, however, that the scandal is not really about the President's private life but his conduct at work - about a pattern of behaviour in which female employees, from Paula Jones to Monica Lewinsky to Mr Clinton's secretary, Betty Currie, were expected to provide favours for him. These included sex and, in Ms Currie's case, acting as cover for a relationship the President was ashamed of and wished to conceal.
We are talking here about Mr Clinton's secret life, the disjunction between his covert, enthusiastic promiscuity - he apparently told Ms Lewinsky he had had 'hundreds" of women before the age of 40 - and his public promotion of himself as a devout family man.
If Mr Clinton is unable to reconcile his religious beliefs with his sexuality, as appears to be the case, then he has a big problem. His solution, to lead a double life which has now been exposed to world-wide ridicule, may not be grounds for impeachment - but it does convict him of hypocrisy and sexual cowardice, as well as placing him out of step with the relaxed attitudes of the nation he still aspires to lead. The Farrelly brothers, as we can see from their latest offering, may not make great movies, but at least they think sex is funny, rather than a mortal sin.