Q. My stepson, who is 18, is planning to marry next fall as soon as his girlfriend turns 16. He is quite immature (all of his male friends have been two to three years younger than he) and this is the first girl with whom he has ever had a relationship.

He lived with us for three years. Last winter he left because we would not allow his girlfriend to move into our home and into his room. He has moved out of state to live with his mother, has dropped out of high school and is supposed to start working soon at a fast-food joint.

The girl lives with her mother and stepfather and has a history of running away from home. Her parents are quite strict: no phone calls during the week and her mother once sat up every night for a week to make sure her daughter did not sneak out of the house. This has made her even more rebellious.

We never forbade our son's seeing her, nor did we try to diminish his feelings by referring to their relationship as puppy love. We have, however, tried to point out the realities of life with respect to the type of job he can expect without even a high-school diploma and the fact that he has so far been unable to support himself, much less a wife and the child they plan to have soon after marriage. We have cited statistics on the incidence of divorces in teen-age marriages and reminded them that both sets of parents married in their teens and were divorced soon after. Of course, they feel that none of this applies to them.

We are hoping that time and distance will mitigate against a marriage next fall, but they have kept in constant contact. From our perspective, it looks like a disaster in the making. Any suggestions you have would certainly be appreciated.

A. The sorrow here is not that two ill-prepared young people are going to get married, but that they feel so lonely and scared and unloved that they think marriage is their only hope.

You're wise not to denigrate the feelings of these young people; certainly no puppies could love as deeply and fiercely as teen-agers. However, their dependency on each other shows a lack of confidence in themselves that is alarming. Fantasies seem to be all they have.

It would be hard to imagine this marriage working out well, but your advice and statistics and arguments aren't going to convince your stepson. To some extent, he does know what he needs--or doesn't need. And while it is necessary to uphold your own standards in your own home, your stepson has a sense of honor too, which he feels he must defend. However misguided his conclusions, he has reached them on his own and he must change them on his own too. With help he may be able to do it abstractly, before he turns marriage into in-service training.

Instead of arguing the merits and demerits of their plan, why don't you accept it, with the proviso that your stepson, and if possible, his fiance'e, see a counselor first so the marriage can get off to a better start. This would be the best wedding present you could give. Your stepson will be part of your lives always; it's time he understands that you're on his side. So long as you make this offer in the spirit of a gift to prepare them for marriage, rather than a trick to stop them, they may agree to go. A psychologist, or that most pragmatic of counselors, the psychiatric social worker, is much less judgmental than a parent, asking questions rather than giving answers.

According to Lawrence Kohlberg, in his important new book, The Philosophy of Moral Development (Harper & Row, $20.95), this Socratic method helps a person clarify his own values and improve them, too. And this is what your stepson needs. Until he can think of others more, and understand cause and effect better, he won't be able to internalize his conscience and accept the responsibility of his actions. This usually happens in the early teen-age years, but it can be much later--or not at all. When an 18-year-old prefers much younger friends, it suggests that he has a way to grow.

By talking his situation over with a counselor, your stepson should be able to find out where he really stands. It also should make him less defensive so he can talk about his own fears. In the process he might realize that marriage is easier if a man (and a woman) achieve certain milestones first: like a high-school diploma, a job and a track record for handling money, so the emergencies of life won't be so overwhelming.

Reader's Response: To the moving consultant who feels that an 18-month-old child has no idea what is going on in a move . . . I beg to differ with him!

My daughter was 18 months old last year when we moved from Michigan to Virginia and believe me, she was deeply affected by the move. She is a sensitive child to begin with, and the new faces in the neighborhood and new locations of all her favorite things--not to mention a new room--frightened her terribly. For four months she clung to my husband or me in the presence of a stranger and screamed if a neighborhood child (or adult) made friendly advances toward her. She was old enough to know her world had been physically altered drastically, but too young to understand why.

In contrast, my 3-year-old adjusted beautifully, thrilled with his new friends and new home.