A Cambridge, Mass., research organization weighs in with the most startling datum. It is that men and women who live together before marriage (judging from a Swedish sample) have a higher divorce rate than men and women who do not. By 80 percent. The only inference given over the news broadcast as to why this should be so is that perhaps those couples who did not cohabit before marriage held the institution in greater respect.

The datum is one of those heavy anti-pop-sociology items that come in from time to time, rather like the finding that Margaret Mead's Samoans were actually a wretched lot with their free sex; or that the state (New Hampshire) that spends the least on public schools gets the highest SAT scores; or that students reeking with sex education in the schools contract more venereal disease than those who do not. It has got to be the hoariest assumption of Free Thought that the very best way to ascertain strategic compatibility between potential husbands and wives is for them to share quarters together as man and wife. Even the orthodox have hesitated to challenge the generality, because it does sound so -- well, sensible, does it not? But suddenly the figures come in, and they contradict that cozy assumption.

Which leads to the question: What is it that a monogamous society is supposed to do to provide what Mr. B. F. Skinner would call ''positive reinforcement'' for the idea of monogamous life? I recently recalled that up until about 35 years ago, the Royal Enclosure at Ascot would not admit anyone who had been divorced. The tradition (a strange one, to be sure, to evolve from the head of a denomination founded by Henry VIII) was first waived to permit Anthony Eden, then the prime minister of Great Britain, into the enclosure, notwithstanding that he had, indeed, been divorced. The reason for the exemption, it was discreetly given, was that in the case of Eden vs. Eden, it was unmistakably the lady who had been aggressively unfaithful. Granted, to apply the old rule strictly nowadays would require the queen to forbid her sister access to the enclosure; and that would cause, in domestic terms, what in political terms we would call a constitutional crisis.

But the question crystallizes: How is monogamy to be encouraged? We know, if only because we remember that Thomas Jefferson told us it was so, that laws are far less important than public opinion, which laws tend merely to codify. What kind of social pressures today survive on the matter of bigamy?

In recent weeks much public attention has been given to a spectacularly successful British gentleman who, like Alec Guinness in the wonderful movie ''The Captain's Paradise,'' keeps two families. The difference being that Guinness' wife in Algiers knew nothing about Guinness' wife in Gibraltar, and the whole point of the story revolved around the discovery by Wife A of the existence of Wife B, and of their complementary social habits.

That movie's plot structure is about as antique as moral austerity at the Ascot Enclosure. If there is a reason today for Wife A not knowing about Wife B, it would have to do with jealousy, not with social toleration.

The gentleman in question evidently keeps two families, in different countries. And this is advertised, by those who write about him, with the same nonchalance with which one would treat someone who had a castle in Spain and a chateau in France. It has been hinted that the gentleman polygamist was denied a seat in the House of Lords because of his provocative domestic arrangements, but the question before the house is, really: Are such arrangements provocative? They are hardly new. Those who notice these things are aware that Mr. H. L. Hunt of Texas had a wife in Dallas (his public wife) and another elsewhere. But, again, the arrangements were highly discreet. Highly discreet because it was acknowledged in those days (1 million years ago -- i.e., before Woodstock) that bigamous arrangements were -- how dare one put it? Anti-Christian is too weak (nobody cares). Un-American? But what's un-American? Didn't we abolish the House Committee on Un-American Activities because we couldn't agree on what was un-American?

All we have to worry about is the gradual evanescence of the family. Twenty-five percent of our families are headed by a single parent, 95 percent of them women. So? So what? is all we hear around us.

1987, Universal Press Syndicate