HERE'S A RIDDLE for you. In South Africa, where the white minority enforces - overwhelmingly to its own advantage - the system of racial separation known as apartheid, one law proscribes marriage and another extramarital sex between blacks and whites. As a result, in such a racially strict society, especially one dominated by God-fearing, law-abiding Afrikaaners, you would expect to find almost no one of mixed blood, aside from the progeny of a few assaults or accidents. Right? Wrong. There are upwards of two million "coloreds." No laws may have been violated more frequently and, you may be sure, at the instance of white men, not black men, than the "Immorality Act" and "Prohibition of Mixed Marriages Act," as they are called.

But these laws represent much more than a boundless white hypocrisy and arrogance. Reaching an area that all civilized societies leave to personal, extralegal choice, they wreak indignity and misery upon the hundreds of individuals each year who are prosecuted under them, and they cast a pall of prejudice upon the non-white communities as a whole. They are, justly, a principal fount of the odious reputation that is South Africa's in the world today.

It is, then, no small matter that within the ruling National Party itself these laws have become a special arena of the larger dispute now developing over apartheid. It pits those who would eliminate the harshest aspects of apartheid in order to preserve a white position in the nation, against those who fear that even limited and controlled changes may impel whites down a slippery slope on which there can be no pause. Several government ministers sensitive to South Africa's international position, if not also to the feelings of its non-white citizens, are urging repeal. The party's hard-core conservatives, still very much in the saddle, oppose repeal. It could, they suggest, "unleash an uncontrollable spate of sexual excesses and dissipation which may be totally abhorrent to all races." The issue is expected to arise in the next session of Parliament.

The South African government crieds out in nationalistic indignation when foreigners suggest that its racial policies render it unfit for international company. This puts on that government a special obligation to take the burden of change upon itself. No one would deny that these laws governing cloices fo sex and marriage go to the heart of the political relationship between the races. If the white society admits it cannot control these particular choices, it will be forced to ask itself how it can claim to control others. But whites can no longer evade these questions. The necessity to confront them is painful but clear.