If you didn't know if you'd been dropped, alien and ignorant, into the midst of it all, you could be excused for thinking you'd just discovered a new form of life. In the mattress area, scores of dangling hands, arms and assorted legs are waving frantically like some millipede.

In utter solemnity, with his arms folded like a lifeguard's, a man sits on a chair, majestically surveying the moving limbs, making sure the sexual ecesses don't get too. . . excessive. Making sure, for instance, that no one walks into the area alone.

There are rules here.

You cannot walk into Plato's Retreat without a woman (if you are a man) : you cannot go into the mattress areas unescorted no matter who you are; you cannot drink, smoke, eat in the mattress areas, Olympic-sized swimming pool or whirlpool bath.

You cannot do heavy drugs engage in prostitution. And yet, as club co-owner Larry Levenson points out, who needs to pay for sex at Plato's? It's free here, in the open here, right under your nose and unavoidable here, once you've forked over your 25 bucks plus your $5 temporary membership fee - an all-inclusive charge for two people, one that allows you to indulge in, among other things unlimited eating, and voyeuring.

"Sex is not the main factor at Plato's," Levenson insists solemnly. "This is a mellow atmosphere, a nice place to be. You come here for complete freedom to do anything: nude bathing, dancing, meeting dynamite people . . ."

Levenson is being very cute here. Sex, as it happens, is the main factor at Plato's, just as it is not exactly a casual aside in several other clubs that have lately sprung up in New York City. Nude bathing, after all, can be easily accomplished in the privacy of your own bathtub. And as for Plato's "dynamite on their intentions:

"Now if people go in for (making love), that's something else. If a boy meets a girl here - that's beautiful. Dynamite."

If a boy meets a girl here, she might be excused for assuming at first that she has been approached by either Fred Astaire or the Galloping Gourmet. There are two basic lines at sex clubs (just as there seem to be two basic lines everywhere). One is, "Hey! Would ya like to dance?" And the other is: "You know, you look lonely/hungry. Can I get you some chicken salad?"

(It's too bad if you're thirsty: The New York Liquor Anthority has recently told Plato's that no liquor can be consumed there, and Plato's is fighting it.)

As it happens, though, you can eat and you can dance at Plato's. As it happens, some people do. But not for nothing is the place an endless length of darkness, furnished with-mattresses, deck chair and - in the rear - small rooms with huge Oriental pillows, and locker rooms that dispense little white towels.

These last constitute the basic appatel worn at Plato's Retreat - little towels tied neatly at the waist of, for the less modest, slung casually over a shoulder. These last are, at first glance, also virtually the only thing that distinguishes Plato's from its first cousin - the singles bar.

But of course, the new sex clubs are more than singles bars.Not much more - but more The new sex clubs are simply the commercial materiaization of Erica Jong's sardonic dream: the zippless encouter, as we shall have to call it. The dream of sex, uncomplicated and unencumbered, among the unnamed. The only thing that sort of sex is not, is unplanned. In the new sex clubs, sex is planned, it is organized, it is zoned, and - like every other rage around - it is highly and exclusively middle class.

Eve with the pendulous bosom, stretches out by the pool and shrugs good naturedly. "A meat market," she remarks, stretching lazily. "Really, what's so incredible about Plato's is that the same inhibitions that exist in the outside world and the singles bars also exist here."

In fact what astonishes at Plato's and its sister clubs is not the shedding of inhibitions (that, after all is by now a century-old moulting process) but the tenacious way form is clung to in the midst of chaos. The discreet charms of the bourgeoisie have simply turned slightly . . . indiscreet.

In an entire of sex-club hopping, which lasted from 10 p.m. to 3 a.m., one is likely to get a lot of proselytizing from true believers, a lot of advances, a lot of defeasive rationales. But the one thing you almost never get is names.

"I'm a cardiologist. How do you think my practice would true if I told you my name?"

(And so now you know where to go to meet a doctor . . . ) Our Greek Heritage

There are by now three fairly wellknown sex clubs in New York, and one in San Francisco. But Plato's with its initial capital investment of $250,000, its two-year history and its more recent Upper-West-Side location has something over all of them.

It is establishment orgy.

That is its strength, that is its drawback, and that finally, is its media value. What used to be done, quite flagrantly, but always privatly in the late '50s and '60s, has now come out of the living room.

What used to be accomplished among friends is now hitting New York magazine, Time magazine, the Village Voice - and by the time any New Wave has lapped against the porous borders of Time, one can be sure that it is neither new, nor any longer a wave. It is by then a sizeable stream.

"Thank God for Screw magazine," Levenson says with a gratified sigh. "They gave us our start."

But one cannot credit either the media of Levenson's silent partner (no names, as usual) with the beginnings of Plato's. In a broader sense, it is the Greeks and Romans who gave us what we have now. Orgies that have been with us since those ancident times have traditionally been explained away by the primmer writters of history. The empire falls. Either Christianity or the vice squad intervenes. But something (it is fondly presumed) happens to restore what we like to call the natural order.

But this has yet happened with Plato's. It's all legal, said Levenson.

And, in fact, this has not happened well before Plato's ever came about.

"Kinsey, in his sexual studies during the '30s and '40s, found evidence of group sex," says Edward M. Brecher, authof of "The Sex Researchers." Brecher, a historian of sex research, also points out that the indulgences of ancient times were markedly different from those of the '60s.

"What made the '60s different is that one saw both husband and wife participating," says Brecher. "Not just a man and a prostitute."

No one, says Brecher, has any idea of how many people are swinging these days. The only thing everyone has observed is that men are more likely to do so than women; that a more egalitarian mood has settled over the proceedings (tne word "wifeswapping" for instance is not in usage); and that "any sentence which begins with the words, 'All swingers . . .' is just false," according to Brecher.

"Essentially," he contuines, "it turns out that jealousy is far from universal. That there are husbands and wives who are quite unthreatened by this kind of behavior. Nobody knows whether jealousy is learned or innate, but there are people who are totally without it."

Even more to the point, those of us who used to take comfort in the belief that sexual excesses were historically the vices of the bored and jaded upper classes must now acknowledge that a sizeable portion of the middle classes are now indulging quite openly and possibly from the same motives in those same adventures - and have been in this country on a sizeable scale ever since the late '50s when a West Coast Lounge opened as a meeting ground for couples who wanted to play more than Gin Rummy.

"I wouldn't want you to think that all these scenes are like Plato's with no intimacy involved, no talking," says Brecher. "That is not the case elsewhere. In Minneapolis there actually is a very close group of couples who were originally church-associated - three of the four couples belonged to the same church; it's very communal."

But Plato's with its maximum capacity of about 600, its lines of patient customers waiting to get in on a drizzly Friday night, marks a giant departure from all this homey, living room entertainment.

Plato's signals the beginning of the mass-merchandising of live sex. The prices it charges are proof of this:

"Twelve-bucks-fifty a person," marvels Levenson. "Can you imagine! In this day and age? No big bucks in this thing."

Oh yes, Larry Levenson knows the value of mass merchandising. Once he was a general manager of a McDonald's. Now 41, divorced, and equipped (like so many men one sees at Plato's) with a large belly, Larry Levenson is a sublime product of the fast-food business, the perfect Big Mac. He has learned, for instance, not to fear but to welcome the competition ("Nobody goes to the same place all the time") even while putting it down ("Fifth Dimensional," a rival New York sex club, he criticizes heartlessly).

He has learned the value of appealing to mass boredom, luring customers with the you-deserve-a break-today siren song: "Sure swingers can have house parties," he'll admit. "But what is it? Five couples? That's just sex. But here - here we have everything. Some people, don't want to see the same people all the time."

He has embraced the fast-food business dream: the prospect of a franchise:

The first Plato's franchise is probably going to be in Washington, D. C. Yup, it looks that way."

And he has inspired that supreme compliment: the imitation. Midnight Interlude, when last seen on opening night seemed a pretty tacky imitation. But as an inevitable product of capitalism and free enterprise, it is probably most appropriate that it should be located in the Wall Street area. The Come-On

On opening night at Midnight Interlude, the blond writer approaches with her male companion.

"You seem like a very nice couple," she offers brighty, as her listeners retreat. There is no such thing as a gratuitous compliment in a sex club.

The writer is a short person, wearing a T-shirt that reads: "A Woman's Place Is in the House. And Senate." And she has, as you might expect, a feminist rationale for swinging, which is clearly what she wants to do now. But first she must convert:

"Women," she explains gravely, "must get more of a male attitute toward sex. We tend to associate anything sexual with romantic expectations and men don't have that hassle. So we have to change to deal with this."

She smiles fondly at the man she came with, who was her mentor. "The first time I went swinging, I made love to 20 people. And now I wonder if I could ever go back to the straight life. The restrictions of fidelity are too congining, if you know what I'm saying. I mean I could see getting married and swinging with my husband."

So why bother getting married at all?

"That's a good point."

While this conversation is going on, two men are making love to one woman in the middle of the disco room, an activity that has succeeded only in stopping the dancing. The writer and her friend are talking, in fact, as if nothing were happening, which they both fervently wish were the case. The trilogy on display centre-stage consists of a grotesquely fat couple and a shockingly skinny man.

The writer's companion makes a face. "The funny thing," he muses, staring glumly at the three centrepieces, "the odd thing is that the people here are defined by the fact that they swing. Not by what they do or who they are, but only by the fact that they swing. That's why I won't give you my name."

His eyes follow the skinny man who is leaving after the performance. Finally, he says, wonderingly, "You know I just saw that movie, 'Saturday Night Fever.' That's what it's like for these people. This their Saturday Night Fever."

But it isn't really.

There is no passion here. The Wallflower

Almost every face at does not start out that way.

It starts out, like your average Sunday School dance, with anticipation. The new faces seem stunned or expectant of fearful. What if you're not asked? What if you ARE asked? What if you ask and nobody likes you? This happens, sometimes.

The lawyer is lonely and bored at Plato's - so bored that (this is not an uncommon reaction at sex clubs) he demands imperiously to be interviewed. People at sex clubs will stop anything for a chance to be interviewed. Anything at all. The lawyer, unfortunately, doesn't have anything to stop.

"My girl friend - she won't come here," he mourns. "She's uptight, I guess. So I got this hooker to come.

This hooker - she cost 75 bucks - she's my present to myself." But his expensive present to himself isn't around.

"Yeah, I know," the lawyer acknowledges sorrowfully. "She's a real robot, I swear. A tough board, and now she's out looking around for some rich guy."

He looks up at the busy crowd, dancing naked, swimming naked. Then his gaze shifts over are two men who are not naked. One, wearing red Doctor Dentons, is making love to a fat lady; while another, wearing a three-piece suit, is looking on.

"So don't you want to know what I think of this place?" the lawyer demands angily. "My opinion of this place is it's nothing special. I like to watch people.

"Now the bad part is it's got a somewhat seedy atmosphere."

The bright red Doctor Dentons leave the mattress room, to be replaced by a naked man wearing socks. Socks are very big at Plato's.

"Yeah, seedy," continues the lawyer. "But put this in your article: It's not a big deal here. Millons of people should come and not be frightened. I mean this should be an everyday occurrence. Next time I'm going to bring my girl friend. I sure wish she were here now."

The lawyer demands a copy of the article. As he will not give his name ("How do you think my clients would like that?"), the impossibility of fulfiling his request is explained to him.

"Oh yeah, you're right." His face falls, then brightens. "Got an idea. Send a copy to: "Mr. D., . . ."

"Send a copy," advises a nameless doctor, to: "Occupant . . .'" Intellectual Swinging

You don't have to be smart to make love to a lot of people, but from the way the clientele talks, you'd think you needed a master's degree, at the very least.

"I gave a Mensa party," a naked lawyer explains pointedly to a naked lady at Midnight Interlude. "The other night I gave a swing party just for Mensa people."

"Oh," says the lady, "I don't belong to Mensa."

"That's okay," he consoles her, "You . . . good."

Behind them, there is a small buffet, which is approached by a tiny gray-haired man.Every five minutes the gray-haired man leaves the gray-haired woman he is with, and grabs a few bagels from the buffet, stuffing most of them in his pocket.

A young couple enters, takes this in, and pronounce themselves disgusted with the low caliber of the clientele.

"Could you die from this place?" the woman demands loudly of her partner. "Could you just die."

"That," her friend replies stiffly, remains to be seen."

Undaunted, the woman takes her complaints to a stranger. "The problem with this crowd," she explains, "is that they're just not intellectual enough." 'Bye, It's Been Fun'

There is a paradox here. On the one hand sex-club regulars like to launch into erudite rationales for their hobby and expect casual encounters to be able to do the same. ("It reminds me of marijuana 10 years ago," remarks Brecher. "When you're onto a good thing, you want your friends to be onto a good thing.")

On the other hand, nobody goes to a sex club to talk. Given the roar of the disco music, talking is virtually the only activity inimical to Plato's. Nor does Levenson seem to encourage it:

"You go to singles bars," he says scornfully, "and whaddya get? All that bull -. How rich the guy is. What he can do for you. Ever met a married man at a singles bar? Nah, of course not. Everyone says he's single. You go home with him - and can't wait to get rid of him - right?

"But here - here at Plato's it's refreshing. No bull . . . No telephone numbers exchange. After a sexual affair, he takes her back to the bar. You don't have to listen to all that crap. You ask the guy - Who are ya with? He tells you. He says, 'I'm here with my wife,' To me that's solid. That's refeshing. No bull . . ."

"Who are you with?" the literaty agent asked a woman.

"I'm here with my husband," she replied. Politely she introduced the man next to her, who looked stoned out of his mind.

The literary agent, new to this scene, and unfamiliar with its mores, persisted in his small talk. "And what do you do for a living?" he asked the woman.

"I work with deaf people." Smiling, she made a few gestures in sign language.

"What does that mean?" asked the agent.

"It means," she replied, shouting over the din, "It means, I want to f. . you.'"