CONVENTIONAL society, once considered the ruthless enemy of unsanctioned romance, has grown tolerant and even gentle toward renegade lovers, most of the violations for which illicit couples used to be condemned have been removed from the books.

So you would think -- or rather Miss Manners would, since her mind tends to wander toward matters of etiquette while everyone else is concentrating on the merely salacious--that such lovers would, in gratitude, show extra respect for the few remaining conventions.

They don't, though. Freed from the necessity of hiding much of their relationship, illicit couples, Miss Manners has sadly noticed, tend to behave as indecently as formally engaged couples. She is disillusioned.

Violating convention, for couples who are married but not to each other, used to consist of such infractions as eating public meals together, looking fondly at each other when witnesses were present or being out of town at the same time.

But we live in pleasanter times now, where the rules are more in keeping with Miss Manners' natural desire to discover innocence lurking everywhere. If she sees a gentleman and a lady holding hands on the street, she presumes a delightful friendship enriching the lives of all concerned, and if she sees them emerging from a hotel at noon, she presumes it is from the conference room.

So then why do they have to corner her and tell her about it?

The proper rules of conduct for improperly paired lovers (sin being no excuse for bad manners) toward others are:

Make as few confidences as possible, and only to people who seem receptive. The natural desire of all lovers to have everyone in the world know about them--based on the assumption that they appear enviable, rather than that everyone is thinking "What on earth do they see in each other?"--leads to a great deal of reckless showing off. We tolerate this in approved lovers, holding back snickers only for the inevitable time when they voluntarily stop cooing in public; in illicit ones, it is embarrassing to many people.

Do not ask others to lie for you. We are all responsible for our own morals, and to ask people to compromise theirs without the advantage one enjoys of being compromised, as it were, is unethical.

Do not put others into situations of dramatic irony, in which they are aware of double meanings in behavior of which others, usually the spouses of the lovers, are innocently oblivious.

Leave borrowed quarters as they were found. Bread-and-butter presents are appropriate for all houseguests, even those who did not actually stay overnight.

Each of the lovers is as bound by discretion, loyalty and secrecy about any privileged information about the other lover's spouse and family as the person who told it was supposed to be.

Miss Manners recognizes that frail humanity is often subject to chaotic emotions and uncontrollable passions. But she refuses to acknowledge that the careless or willful desire to violate the rules of etiquette is one of them. MISS MANNERS RESPONDS

Q. I am very lucky to have a loving mother who has been giving me some of her family heirlooms through the years.

My husband's two adolescent daughters have been living with us for the last year and "eye" each item as it enters our home or is used, often expressing their choice for (future) ownership.

We are planning to have our own children in the near future. The girls still have close relationships with their mother and both sets of grandparents. They are not close to my family, although they do live nearby.

Is it correct to believe that the girls should expect their "heirlooms" to come from their mother or grandparents? And that it is my right to save or give these gifts to my children without hurting anyone's feelings?

A. First things first. If you teach your stepdaughters some manners, you will be leaving them a more valuable heritage than your mother's jewels.

The key point here is not what will happen after you are dead, but the fact that you are alive. No matter how valuable the property, no matter how clear the lines of inheritance, it is incredibly rude to speculate on one's spoils in the event of the death of the person with whom you are discussing the matter. Even a crown prince does not (if properly reared) try on his father's crown before the father is certain not to sit up in bed and object. Only the person with the property can bring up the subject, and then, the proper response from the heir is, "I can't bear to think about it."

Your response to your stepdaughters should be a pleasant, "Well, I'm not planning to die for a while yet, so if you don't mind, I would rather not hear this discussed." You may then do what you wish and leave them to find out your decision when your will is read.