Q.Some time ago you wrote about "sex and the single boy," pointing out his sexual responsibilities.

I was disappointed with your assumption that all male teen-agers are heterosexual. This is certainly not the case. You should have addressed some of the questions that face gay teen-agers, or at least mentioned some self-help books.

A. Your letter led to much researching, soul-searching, and a greater appreciation of the growing-up problems of gays and lesbians.

It is stunningly clear that teen-age homosexuals -- boys and girls -- need one attribute above all others: courage.

To grow up different, and to know it, is very hard, especially between the ages of 7 to 16.

In these years, when conformity is the hallmark, children constantly compare themselves to each other and wince at every flaw they think they see. If their bodies don't match their feelings, a true despair sets in. It is the ultimate in noncomformity.

Parents expect their boys to be boys and their girls to be girls, and so do teachers and classmates and everyone's Aunt Tilly. This only makes the hurt worse.

Few gay teen-agers dare to tell their families about it, until they leave home, for fear of losing their approval. And yet the price for this is high. The claustrophobia that comes from living in the closet is suffocating and the deception attacks their sense of honor. People have a terrible need to tell the truth.

There's no doubt about it. It hurts children to grow up gay, and it keeps on hurting until they can admit it to the people they love best: their families.

This generally doesn't happen until the late teens, and some experts say this is soon enough. A teen-ager who announces too young, without being absolutely sure, would set up unnecessary problems.

However, most gays and lesbians know they're different with the first stirrings of puberty and even before.

It appears to be the luck of the dice.

According to today's thinking, homosexuality can be neither caught nor taught, nor can it be reversed. By the time kids know which sex they are, the die has long been cast. There is no choice. No one, however, knows why.

The American Psychiatry Association took it out of the mental illness category in 1971, and in 1981 the Indiana University of Sexual Research Institute (the Kinsey Institute) and the National Institute of Mental Health eliminated many other theories in an 11-year study of 1,600 homosexuals and heterosexuals. It found that homosexuality is inconsistent with all of the traditional psychological or sociological explanations then in use.

Some scientists believe that early puberty may condition a child for homosexuality -- since pre-teens are still hanging around their own sex most of the time -- but most current research has a biological focus. If heterosexuality has a physical base, homosexuality may have, too.

Hormonal studies have just begun; heredity studies will be published soon; and last year psychobiologist Brian Glandue reported a neuroendocrine difference between straight men and gays. No one, however, yet knows whether this difference is the cause or the effect of homosexuality or if it's particularly relevant.

To Adele Starr, president of the Federation of Parents and Friends of Lesbians and Gays (FLAG), the only consistent aspect of homosexuality is its inconsistency. It appears equally in all ethnic groups and in all families: rich or poor; one child or many; whether the parents are single or married; if the mother or father was strong or weak.

Scientists believe homosexuality has always existed in the population to about the same degree -- about 4 percent who only have homosexual experiences, and another 6 percent who prefer their own sex but have experimented with the other (some doctors feel the total is lower, while the gay community feels it's higher).

However, none of this matters as much as the way the family -- and the child -- handles the situation.

Homosexuals deserve the same unconditional love from their parents as any other children -- for they are sons and daughters first -- and they need the same respect from their siblings and classmates and teachers.

For their part, they need to be patient with their families, who haven't had as long to absorb and accept the information. And they ought to be discreet, so others will see them as people, not homosexuals.

And they need to be strong enough to endure the taunts of others, which will always be part of their lives, although how much a part will depend on how much tension they invite.

Above all, young lesbians and gays need to be to be as sexually responsible as any other teen-agers. This means that sex shouldn't be indulged in too soon. High school students -- straight or gay -- may feel ready for sex, but few are ready for its consequences.

Sex is also too precious to be squandered on chance encounters. Promiscuity is not the sign of low self-esteem, but it inevitably lowers it more. Teen-agers -- straight or gay -- have to realize that sex only reaches its full joy if it's the result of a strong and loving relationship.

AIDS is another real danger. Any responsible homosexual, of any age, should beware of pick-ups and should use protection with anyone, unless they have had a long-term, monogamous relationship.

Casual sex is also emotionally dangerous, for it can hurt someone else if it invites affection that is then cast aside.

All of this amounts to kindness, to oneself and others.

Almost every city has a strong support system for gays. A gay bookstore is a good place to find out about it. There are many good books there, too, for the sexually confused or worried teenager -- and their families -- as well as at a large public library.

Among the best: A Way of Love, A Way of Life by Frances Hanckel and John Cunningham (Lothrop; $7.95); Are You Still My Mother? by Gloria Guss Back (Warner; $7.95) and A Family Matter by Dr. Charles Silverstein (McGraw-Hill; $3.95). There's also FLAG's small, fine, free pamphlet in five languages, About Our Children. Send a long, stamped, self-addressed envelope to Box 24565, Los Angeles, Calif. 90024.

Questions may be sent to P.O. Box 15310, Washington, D.C. 20003.