Teachers and principals searching for effective material for their sex education classes might want to tape Saturday night's Bill Moyers special, "The Vanishing Family -- Crisis in Black America."

The two-hour CBS production contains lessons for just about everybody. It is a reminder to the social theoreticians that we really do need to look at the way we do welfare; it is an alarm bell telling church and community leaders that they had better get busy rebuilding the collapsed value system in the black ghettos, and it makes clear the inability of government, no matter how generous or compassionate, to deal with the problem.

But its greatest value may be as an instrument of sex education for the young people who are playing at sex, with hardly any recognition of what a dangerous game they play -- for them and for America.

And especially for black America.

"Single parent families," Moyers reminds us, "are twice as common in America today as they were 20 years ago. But for the majority of white children, 'family' still means a father and a mother. This is not true for most black children. For them, things are getting worse. Today, black teen-agers have the highest pregnancy rate in the industrial world, and in the black inner city, practically no teen-age mother gets married."

Yes, there are strong, successful black families; the "Huxtables" of the Bill Cosby Show represent a reality that gets too little play on television. But the Moyers special is about a different reality: the almost irretrievable collapse of the family in the black ghettos.

The alarming increase in the number of teen-age mothers is both the result and the accelerator of that collapse.

But as "The Vanishing Family" makes clear, the inner-city teen-agers themselves seem wildly oblivious of the crisis. Few of the young mothers interviewed planned to have babies, but hardly any of them took any precautions against it, not because they are ignorant of birth control but because they are ignorant of the implications of having babies before they are ready -- psychologically, educationally or economically -- to be parents. The young fathers display an unsettling combination of pride in having fathered babies and utter indifference to the babies themselves.

"Well," offers a young man who has sired six babies by four different women, and who supports none of them, "This is something that I've done. Just like the carpenter, here's something that you've done. You can see what you've done if it's anything. If you don't do nothyou know, what your life was, you know, it was to you."

And how does he expect his children to survive without any support from him?

"Well, the majority of the mothers are on welfare, and welfare gives them the stipend for the months, so what I'm not doing, the government does."

That's part of the breakdown. The other, in many ways deadlier, part is that the young women expect -- even unconsciously encourage -- just such an attitude.

Moyers asks one mother whether she wants her own children to get married.

"Sure," she says. "Especially my daughter. My boys, they'll probably be -- whatcha call it? -- free-lancers."

If it seems as though no one has given these young people any conception of what being a real father entails, it is because no one has. Often themselves products of teen-age pregnancies, many of them have never experienced the presence of a normal, functioning father. The idea of a husband as a provider, disciplinarian and role model for his sons seems not to occur to them.

A part of the reason may lie in the stupendously high unemployment rate that renders inner-city young men superfluous except as intermittent lovers. Part of it may be the changed moral climate that renders out-of- wedlock parenting free of moral censure, and part of it may be welfare's usurpation of the father's breadwinning role.

But as "The Vanishing Family" makes alarmingly clear, welfare can provide money but it cannot teach boys how to be men. It's something the rest of us will have to do, starting right now. Indeed, for much of the growing underclass, it may already be too late.