FOR EVERY TWO marriages in the United States this year, there will be a divorce. The number of marriages is rising because of the increasing numbers of young people stepping across the threshold into adult life. But the number of divorces is rising a good deal faster. The statistics came pouring out of the computers at the National Center for Health Statistics. They are, in effect, the returns in a continuous referendum on attitudes toward life in general and families in particular - matter much too private and delicate for any public vote.

Everybody knows that these trends have been running for some time, but the speed of the change continues to be astonishing. As recently as the mid-1960s, there were almost four times as many marriages as divorces every year. In 1970, there were three times as many. Now the ratio is actually a shade under two to one. Since the average divorced couple has one child, the number of Americans involved in divorces this year - wives, husbands and children - will be close to 3.5 million. That's twice the number of a decade ago.

The most influential of all social statistics, the number of births, is moving the other way. The fertility rate in this country - the number of births for every thousand women of child-bearing age - reached its postwar peak of 123 just 20 years ago. Currently, it's fallen to 66. The girls who grew up in the big families of the last generation - the station-wage families, they were called - have turned in overwhelming numbers against the kind of live their mothers led.

The children of those big families, now old enough to vote, have brought with them into American political life a sense of oppressive physical crowding. It gets expressed in the diverse movements to protect the enviornment, to limit economic growth, to hold down population and conserve resources. The solitary sports - hiking, mountaineering, running - have turned into growth industries for the equipment manufacturers. The United States can't really be called crowded, in comparison with most other industrial countries. The crowding is not a fact but a feeling, which makes it much more important to public policy than any arithmetical computation of population per square mile.

As for death rates, there's a curious disparity here. The computer reports a steady drop in infant mortality. In the mid-1950s, about 26 babies died out of every 1,000 born. Now the deaths are down to 15, a striking reduction. For the rest of the population, there's hardly been any change at all. Twenty years ago the death rate was 9.3 per thousand; not it's 8.9. A physician would be tempted to explain it by observing that, unlike the rest of us, a newborn infant has no bad habits and isn't in a position to ignore his doctor's advice.

The federal computers offer the statistics without comment. But, taken together, the numbers become sketches for a portrait. Economic necessity no longer holds families together. On the contrary, prosperity permits Americans to pursue a new passion for privacy and separateness, even at a certain toll in loneliness. The American syle of living notably heart diseases, as fast as medical progress can push down the rates for others. The only exception, the computer says, is that part of the population that is still too young to climb out of its crib and join the rest of us.