DEAR TEEN-AGED "BROTHER:" Let's level about what happens when you get a teen-aged girl pregnant.

In effect, you "stop" her life.

That means that, because increasing numbers of these young girls are deciding to keep their babies, the natural maturing process that transforms her into a young adult is abruptly ended. She becomes saddled with awesome responsibilities before she is emotionally equipped to handle them.

It's a mind-boggling proposition that she rarely understands until it's too late.

A baby at first may seem to her almost like a doll. Especially if she's a poor girl with so little in life to hope for. But reality soon strikes. There is no escape from the staggering economic burden of having responsibility for another life.

And it is no fun for a baby to try to raise a baby.

Folks like me sometimes look at the "sociology" and shudder. That's a rather big word for saying that births to unmarried girls and women have increased 50 percent in the past 10 years. It's not just a D.C. problem and not just a black problem; nationally, the increase in births among unmarried teen-agers is significantly greater for whites than blacks.

But that's nothing to take comfort in.

Because in Washington in 1979, 55.6 percent of the nearly 10,000 babies born here were born to unmarried women. About half of those babies were born to teen-agers.

Now I know there is a new willingness among women to have children out of wedlock, but I am not talking about adult women making responsible adult decisions.

I'm talking about teen-agers, three-quarters of whom don't set out to become mothers.

Frankly, I'm disturbed that younger children are becoming increasingly sexually active and that this activity is occurring at ever younger ages. Girls in high school and younger should not subject themselves to the traumatic consequences of an unintended pregnancy.

But what is too frequently forgotten in this tragic equation is that young men like you share in the responsibility. All too often, you take refuge in the age-old rationale: "We don't get pregnant, so we don't have to worry."

So, while these young girls try to patch up their lives, you go about yours as if nothing happened.

In my day, when a girl made a "mistake," her parents made her marry the boy. These were called "shotgun" marriages. But if changing times have brought at least one improvement -- increasingly girls today are deciding not to marry their babies' fathers if they do not love them -- the old problem remains. The girl often becomes "damaged goods" in the eyes of those men who say they do not want a ready-made family.

If this girl once had a dream, it now must be buried or radically changed. College or vocational school must be put aside. The "love child" she bore in her ignorance and naivete has yielded an unforeseen burden which frequently she cannot bear.

This is a very delicate cultural issue. For many men, black men in particular, manhood means the ability to produce a child. Some feel that racism has stripped them of so much that about the only thing left to them is their primal pride in producing children. A few years back, some blacks even saw birth control as another form of genocide, another white man's ploy to weaken black power by reducing their numbers. Richard Pryor captured these feelings in a bitingly poignant anecdote about a black man who is asked by a white man: "Why do you guys always hold your crotch?" "Because you done took everything else," the black replies.

But who, really, is being hurt now?

Close to half of all the black children in this country live in desperate poverty. Such poverty is the major cause of mental and physical handicapping conditions in children. A black baby is twice as likely as a white baby to die before it has lived a year. These shameful facts are not your fault alone, brother, for they speak to the inequality of opportunity that still haunts large numbers of black children. But the point is, we're talking about dire consequences. Not only for the girl but for the baby, and society, too.

What can you do about this problem?

You can undo the conditioning that society has imposed on you and learn more about contraception from places such as Planned Parenthood's two male contraceptive clinics, one in downtown Washington and another in Southeast Washington.

You can talk to your teachers and counselors about the subject.

You can think a little more about the consequences and talk to your partner about the contraceptives she's using, instead of simply assuming they are being used.

You can consider the contraceptive options available to you.

Some congressmen and conservative politicians want to make abortions, sex education and family planning and contraceptive planning even harder to get than it already is. These kinds of threats to individual and civil rights could not come at a worse time. But your generation still has the responsibility to build on the contributions of the others. I think you're ready to take more responsibility for your actions even as the rest of us keep pressuring society to open more doors of opportunity.

But you've got to do your part, too.