The secret adults have best kept from the young is that it is perfectly proper and respectable for single people to date as many other single people as they wish.

You may not think that the lust for propriety and respectability races through the blood of teen-agers, but Miss Manners knows better. The unfortunate convention at high schools and colleges now is heavily on the side of narrow fidelity, even if that is of short duration. Dating around, as it were, is discouraged.

You can perhaps imagine how surprised Miss Manners is to find herself in the position of advocating promiscuity, even a mild, social form of promiscuity. (As she is not in the habit of peeking into parked cars or dormitory rooms, she is blithely unaware of any other kinds of behavior that may be so labeled.)

However, she believes that youth would be better off if this secret got out, and young people discovered that there is nothing immoral, disloyal or generally reprehensible about simultaneous courtships.

She will endeavor to show that this stand is actually on the side of morality, rather than against it.

Traditionally, etiquette has never recognized any form of tie between a lady and a gentleman other than that of being friends, engaged or married.

In modern society, a sub-marital category has been added, which includes couples who actually reside together. (This classification, necessary only because of society's legitimate interest in knowing who should be treated as a couple when invitations are issued, suffers from inadequate nomenclature. Miss Manners periodically puts out a call for a public term for this arrangement and receives many answers, ranging from the indignant to the unbearably cute, but has never found a satisfactory solution.)

Let us, however, leave that problem for another day, as the youth problem is one of people who "go together," rather than of those who stay home together.

Among them, the common pattern is to have a series of exclusive arrangements, taking care to break one off just as another takes its place. As Miss Manners is delicately given to understand it, the terms are that romantic privileges are exchanged for the even more coveted license to complain ad infinitum about one's terrible parents, rough childhood, unfair teachers, etc.

Such bonds may, of course, be dissolved and they frequently are, but not without a great deal of explanation, recrimination and hurt feelings. The breaking up is often treated more seriously than the original romance, and a lot of teen-agers are pitifully bruised at this time by being subjected to hearing how objectionable they are from the very people they most trusted and admired.

The standard is, in other words, a rather dreary form of serial monogamy, followed by the pain of a simulated divorce.

Miss Manners believes that such a pattern is both unsuitably restrictive and unnecessarily hurtful. If no obligation to exclusivity is acknowledged, many of these disadvantages would disappear. She believes that much unhappiness could be spared -- not all, of course; what would courtship be without suffering? -- by barring any such arrangements as "going together," "going steady," "going steadily," or going anywhere else that obviously doesn't lead anywhere.

Historically, those concepts served various purposes:

1. People who were freer with their favors than they had been brought up to consider proper salvaged their consciences with public recognition that they were involved in reciprocated True Love.

2. Implicit in the terms was the reservation for company when it would be most needed -- holiday parties, proms and such. In Miss Manners' day, things were so bad that an articulated reason for getting married was, "I'll never have to worry again about getting a date for New Year's Eve."

3. When early marriage was common, this was a way of getting used to monogamy in stages.

These unfortunate circumstances are less common now, and the only advantage supposedly left in tying someone up with pre-engagement declarations never did work. The decent person will never try to hold to a promise someone who is no longer in love -- and the decent or indecent person who wants out will get out.

But Miss Manners' real reason for opposing the custom is basically a moral one:

At some time in life, everyone makes the astounding discovery that half the world's population is of the opposite gender. This is always received as amazing news -- which, indeed, it is -- and gives rise to all kinds of exciting ideas.

Miss Manners favors allowing people in their teens or 20s to make this discovery, unfettered by the responsibilities of exclusivity. To dash from one person to another, reveling in the wicked pleasure of infinite possibility, is best accomplished, it seems to her, during youth.

Miss Manners is tired of listening to the pathetic tales of those abandoned in middle life by overgrown boys or girls who have only just made that discovery.

Q. I need to know whether I am "old money" or still classifiable as "nouveau riche."

My mother thinks I am old money because I am three generations removed from the one who made the money. My boyfriend's mother thinks I am nouveau riche, because the person who made the money is within recent memory and, worse, might show up at the wedding.

The problem is my boyfriend's grandmother, an elderly matriarch who screens all prospective marriage candidates. She has made it clear that she does not wish to be introduced to nouveau-riche girls. My boyfriend and his mother are flexible about these things, but we would all like to accommodate granny particularly since, although granny cannot remember who made the money, she has most of it.

A. Miss Manners regretsouveau riche. Nouveau riche is an attitude, rather than a date on the bank account.

Only the nouveau riche worry about the age of other people's money. Everybody else, including old money, merely worries about money.

Q. I have always believed in helping out friends whenever I could. A friend of mine recently was in the hospital concerning a back injury she incurred a year ago. She has not been able to work more than two months since it happened.

As a favor, I took her cat for the three weeks she was in the hospital. Her cat broke a statue that was valuable, both monetarily and sentimentally. It was a gift from my mother when she was in Spain two years ago, and because it is broken, cannot now be considered a future heirloom.

I had taken great care in placing it where it was, when I received it, so nothing could happen to it.

My friend does not have money because of her inability to work in the past year, but she hopes to return to the job market some time.

My question is: Should I place our friendship above the broken item?

I wouldn't ask if this were some knickknack, but it isn't. Is it right to expect her to replace it with a comparable likeness that can be bought in this country? Perhaps it could be done with the understanding that she could take a period of time to get herself straightened out.

Or should I tell her not to worry? She has made no commitment to anything, and I get the feeling she expects sympathy because of her problems.

A. Miss Manners is having some trouble with, one, understanding what a future heirloom is, or how a generally available statue could qualify; two, believing, in the light of what happened, that you took sufficient care in placing the statue out of harm's way; and three, accepting your description of your charitable nature.

If your friend's claim to sympathy, on the basis of injury and unemployment, strikes you as weaker than your claim to sympa- thy for a broken statue, there is not much here in the way of friendship.

Nevertheless, Miss Manners urges you, for appearance's sake, to place this friendship above your loss. No one else will sympathize with your having taken in a cat without thinking to protect your property against catlike activity.