South Africa is to abolish its laws prohibiting marriage and sexual relations between whites and nonwhites. The laws were vicious and racist in content; they blighted countless lives. They were flouted, in practice, by whites: the Colored or mixed-race minority is the evidence. For ending its intrusion on matters elsewhere left to personal choice, South Africa's white ruling class hopes to win some respite from the pressures bearing in upon it. In selling the change to alarmed constituents, however, it concedes the damning truth that its purpose is not to undo apartheid but merely to dismantle one of its less essential but more politically burdensome abuses.

If the government is determined to treat people humanely and to cultivate family values, it must go on to abolish the pass laws, by which apartheid undertakes to provide cheap black labor to whites without inconveniencing them with the nearby presence of large numbers of blacks. The needed black worker is allowed to work in a particular city, say, but his family must live hundreds of miles away in an authorized black area.

But it is not going to be enough even if the regime abolishes the pass laws tomorrow. A current is running that will eventually, somehow, sweep out apartheid. Petty modifications take much white political exertion but do not come near meeting the expectations of nonwhites to be treated with dignity. Increasingly, blacks see the pursuit of dignity not simply as the receipt of ameliorating dispensations at white hands. It means the sharing of political power in their own land. Nothing else will do. Nothing else should do.

Secretary of State George Shultz this week defended, on pragmatic and moral grounds, the Reagan policy of "constructive engagement" in southern Africa.imes the administration has seemed and has been insensitive to black rage and black claims. But it is right to try to bring along the white establishment, which holds all the real power.

The administration functions as a "good cop," carrying American criticism of its policy to white South Africa and warning that it can't hold off the "bad cop" much longer unless further changes are made. Like it or not, the administration's critics play that "bad cop," denouncing constructive engagement and insisting on censuring, isolating and punishing South Africa. The combination is not working fast or well enough, but it is probably working better than any feasible alternative.

There is a risk in the economic sanctions now being debated in Congress. A strong bill would dramatize American impatience but might also upset the good-cop/bad-cop partnership, leaving white South Africans the more defiant and black South Africans the poorer and more frustrated. That is a formula for greater misery and strife.