JIM PETERSON, alias The Playboy Advisor, has read all those articles about "The Death of Sex" and "The Rebirth of Romance," and he is not impressed.

"I'm beginning to think romance causes all of the problems in the world, not sex," says Petersen. "People get pregnant because it's unromantic to put on a contraceptive. Sex didn't make them pregnant. Romance made them pregnant."

It is perhaps a tad early in the day for this formidable hypothesis, or for matters of the flesh generally. But such is Jim Petersen's crowded schedule of interviews that even as the inbound commuters bump bumpers out on Connecticut Avenue, just a picture window away, he is huddled over coffee and orange juice at the Mayflower Hotel, discoursing brightly on the do's, don'ts, risks, rewards, verities and falsities of that mysterious arena of human relations known as sex.

He discourses with the weight of 10 years' experience as a sex columnist behind him, and with the results in hand of what purports to be the world's biggest sex survey--bigger than Redbook, Cosmopolitan and Ladies Home Journal, just to mention a few of the big ones. And the Playboy survey is not only bigger but, to listen to the man who engineered it, scientifically superior.

His puckish smile, soft voice and L.L. Beany garb make Jim Petersen a rather unexpected spokesman for the Playboy Ethos. Times have changed, however, and things being the way they are in 1983--things like the Women's Movement, the emergence of "vulnerability" as a male virtue and the rates of certain venereal afflictions--the old ethos may not strike everybody these days as positively the last word in matters romantic. Which may explain why Playboy has gone to the computer in quest of hard data to support its view of things, and why it has given this low-keyed, mustachioed 34-year-old the mission of delivering the data to the world.

So when Petersen speaks of sex, let it be understood that he speaks for more than 100,000 Americans who have been asked 133 probing questions ranging from "What's the best moment in intercourse?" to "Have you ever had sex with more than one person at a time?" to "How do you let your partner know that you're interested in sex?"

"I've been living with printouts for five months now," says Peterson. "And a million numbers. I am Tron. I am part of the computer."

He is Tron, but he is a man too. He is a man who describes the Playboy Sex Survey as "neat" and considers himself very fortunate to have been at the center of it.

"It's like sitting in the middle of the Super Bowl, and saying 'All right, how many times a week do you do it?' " he says. "I have been writing about sex for 10 years and I thought I was tired of it. But when I started going into the printouts I learned something new every hour. Again and again, you just go, 'Isn't that neat?' 'Isn't that amazing?' "

A few of the findings, which are being published in installments in the magazine:

* Although people "are doing more of everything . . . the progress of the sexual revolution has not resulted in sexual anarchy." In the same vein, "mainstream sexual experimentation" does not inevitably lead to "whips and chains."

* "Women have closed the gap, and then some" when it comes to sexual freedom.

* "For both men and women, experience contributes to a good self-image." And so, to the question "Does promiscuity lead to disillusionment or despair?" Playboy can answer coolly: "The evidence suggests not."

In short (and in Playboy's own words), the results of its survey "celebrate sex, love and lust."

And this is the Playboy computer talking. This is a Playboy-hired team of sociologists and statisticians from Northwestern University talking. This is 65,396 Playboy-reading males and 14,928 Playboy-reading females (not counting the illegibles, the incompletes and the defaceds) talking. And what they all have to say amounts to nothing less than a resounding affirmation of everything Playboy magazine has been saying for 30 years.

Neat, isn't it?

Now, as Petersen will be the first to acknowledge, it is possible to question the scientific rigor of the survey. It is possible to suggest that Playboy's readers are simply regurgitating back what they have been feeding on lo these many years. It is possible to point out that a number of other magazines have run sex surveys recently, emerging with conclusions equally in tune with their editorial philosophies.

Ladies Home Journal, for example, surveyed 83,000 women and found that the sexual revolution's "most significant effect has been to enrich the love life of the American wife."

Redbook surveyed 26,000 women and men and found that "good communication with one's partner is the factor most strongly related to having a good sexual relationship."

Cosmopolitan surveyed 15,000 women and found that "although the Cosmopolitan girl is sexy and wild, she's also romantic and conservative" and subject to "longings for vanished intimacy, and the now elusive joys of romance and commitment," which could foretell a coming "sexual counter-revolution."

But the Playboy survey--billed as "a continuing report on the state of the sexual union"--is not to be confused with the rest, not according to Petersen. It may not be a profile of "the average American," he concedes, but it speaks for "the veterans of the sexual revolution," who, no less than Vietnam's veterans, "have been waiting for their parade."

While the listener is left to conjure up visions of valiant troops baring their war wounds, Petersen continues: "We make no bones that we are a magazine for people who are interested in sex. We don't apologize for that. We are talking about people on the cutting edge of sexuality, okay?"

Okay. Ripping right along, Petersen launches into an analysis of the pre-Playboy sex surveys, most of which, he says, divided sex into "premarital," "marital" and "extramarital," thus denying single people (like Petersen himself) "the status of having a life style."

The Playboy survey, needless to say, corrects that problem. It doesn't use the word "premarital" at all. Instead, it takes what Petersen terms a "neutral" look at all life styles, concluding that "a wedding ring is not a sexual aid" and that, contrary to "one of the entrenched stereotypes in our culture," singles do not live a "catch-as-catch-can sexual life."

Petersen has even deeper quarrels with Shere Hite's recent study of male sexuality--"if you consider that a study."

"She is giving sex a bad name," Petersen complains. "I didn't find myself in that book, and none of the men I knew found themselves in that book. You have 500 pages of 'Why I don't like sex,' 'What is wrong with intercourse,' 'Why I hate women.' And I'm sitting there saying, 'Wait a second!' "

But what about the reliability of sex surveys in general? Do people always, for example, tell the truth?

"Why would anyone lie on an anonymous questionnaire?" Petersen replies. "Janet Lever, the sociologist who helped us with this, said that an anonymous questionnaire is a confessional--it's today's confessional."

Of course, academics have criticized Playboy's probability sample. But Petersen isn't fazed. "I say, 'Fine, we admit all of that,' " he says. "Their ideal test would be to kidnap a hundred thousand people at random, lock 'em in a room, hook 'em up to lie detectors and ask them about their sex life. Now that would be scientifically pure but it's not likely to happen in this lifetime."

Besides, the Playboy survey was designed to "put a witness into the bedroom." It did this, Petersen explains, by asking men and women separately to reveal such intimate statistics as the length of time it takes them and their partners to reach orgasm. "And you will find that those figures are within microseconds of each other," he says proudly.

The underlying lesson of the Playboy sex survey, per Petersen, is that sex is "a multiple-choice test for everyone, and there is no right answer."

"It used to be so simple 25 years ago," he recalls. "It was a single-answer society. The lesson of the sexual revolution was 'You have permission to choose and experiment and find what fits yourself.' "

And as you choose and experiment, Playboy magazine--which Petersen calls a "Whole Earth Catalog for Urban Males"--will always be available for consultation. It will always be there proclaiming, "Here's everything that's available. Here's the state-of-the-art sex. Here's the state-of-the-art merchandise. Here's the state-of-the-art sex survey." He chortles. "And you pick and choose. It's always been an offering, not a manifesto . . ."

But doesn't Playboy encourage a certain smorgasbord approach to life in which it isn't always easy to distinguish between a sex partner and a stereo? Isn't the catalogue metaphor a kind of program for living in itself? Doesn't the Playboy creed include a certain element of, ah, consumerism?

"No, we're into transcendental meditation," Petersen says soberly. "We're into yoga and the spiritual values that will sustain us in this declining time . . ."

A smirk emerges beneath his mustache.

"Sure, informed consumerism," he says. "It's a joy to play with the right toy. A stereo can change your life more than just about anything else can. It creates an environment around you, right? So don't you want to be alive in this decade? Don't you want to see what this decade has to offer?"

True, one of the things this decade has to offer is the much-vaunted "Rebirth of Romance." This is a phenomenon Petersen thinks he understands, as much as he may deplore it. People "find themselves at 30 or 35 having slept with 19 or 11 or 150 lovers . . . and suddenly they realize that, 'Boy I really don't want to go out and sample a stranger tonight!' "

But that's no reason to assume that promiscuity is bad for people, he says. It just goes to show that "people learn from experience."

So instead of "giving sex a bad rap," those who decide to change their habits should simply say: "Oh, I must have learned something about myself. Now I can make informed choices. I have discovered that I prefer blond Olympic decathlon stars to short brunette stockbrokers, or I prefer these activities to those activities . . ."

And whatever you do, whatever you try, Petersen offers himself as proof that "there is no such thing as too much information."

"I know more about sex than any other person in America right now," he says, "and yet at heart I am still shy and awkward. I am still astonished by the mystery. I still fall in love."