ONE OF THE BEST things Mayor Marion Barry has going for him is the City Council. No sooner did the current council extricate itself from the estate tax fiasco, than it plunged headlong into adolescent sex, approving a bill that would, among other things, legalize sex between children under 12 as long as the children are no more than two years apart. Sex between children 12 and over would also be legal as long as one partner is not more than four years older than the other. Thus, the City Council, acting with the statesmanlike posture that everyone's doing it anyway, so why not make it legal, is seriously considering legislation that says it is fine for a 16-year old youth to have intercourse with a 12-year-old girl.

This, of course, is nonsense. Council member David A. Clarke, the author of the council's latest legislative embarrassment, attempted to give it some standing by announcing that "teen-aged sexual activity that is consentual is not an area for criminal law." If you swallow this, then the next gulp is going to come when the council decides that incest is not an area for criminal law.

The council's actions come at a time when a number of states have lowered the age of consent for teen-agers' sexual activity, a trend that is responding in part to the reality that teen-agers are more sexually active than ever before and that they are more sexually active at a younger age. But this increased activity has hardly resulted in the jails being crammed with adolescent sex fiends. On the contrary: only a handful of cases involving carnal knowledge of an underage person is prosecuted here each year, with prosecutors taking the sensible approach that some young people are old enough to be responsible for their actions while some young people are taken advantage of.

The sexual activity of teen-agers is an area best left to families to tackle, but as has been amply demonstrated over the ages, most families choose to tackle this thorny question by ignoring it and hoping it will go away. Consequently, society has intervened and attempted through sex education and publicly financed family planning clinics to guide the sexual behavior of adolescents in what society believes is a socially beneficial direction.

Up until the last 15 or 20 years, it was widely believed that teen-age sexual activity was irresponsible because the girl might get pregnant and either have to get married or give birth to a child for whom she could not assume responsibility. As society became more sexually permissive and birth control became more avilable, tolerance of premarital (read teen-age) sex grew. cAnd parents, instead of being able to tell teen-agers that they shouldn't have premarital sex because they would get pregnant -- a message that, honored as it was in the breach by older generations, nevertheless was marvelously clear-cut -- had to tread delicately through all the pros and cons of such behavior with children, often not being able to provide them with much guidance at all.

But parents, in discussing these questions, at least have hd a sense of what society considered to be correct and that sense was something they can communicate to their children. They can tell them that society considers sexual activity among young people to be so destructive both to the children involved and society at large that government has laws against it. A parent can tell a child that society has reached a consensus that children his age are neither emotionally mature enough nor financially responsible enough to handle the consequences. Not only are there moral and financial reasons, there are also health resons, and it is one of government's proper roles to protect the health of its citizens through criminal laws. Every bit of medical evidence indicates that young teen-age mothers run the greatest risk to their own health and that of their children, and that young teen-agers who use such reliable birth-control methods as the pill may be tampering with their health in the long run.

There are, in other words, sound reasons for having laws governing sexual activity of children and adolescents. They are laws that are there for their protection as well as for the protection of society. Sixteen may be an unrealistically high age of consent in Washington. Perhaps the council wants to lower it to 15 should it conclude that most 15-year-olds can grasp the full consequences of their actions. But to legalize sex between the very young is not only socially irresponsible, it is downright destructive. Carnal knowledge laws may not land a lot of young people in jail, but they are one more way of keeping some of them out of trouble.