MARRIED PEOPLE. Staying Together In the Age of Divorce.By Francine Klagsbrun Bantam. 319 pp. $18.95.

WE ALL KNOW why marriages fail: infidelity, tragedy, drugs and drink, indifference, bickering and boredom. The surprise, therefore, in reading Francine Klagsbrun's fascinating study of long-lasting marriages, is that the couples whose marriages endure, do so, often, despite the same domestic agonies that kill off half the marriages in the United States. Married People: Staying Together in the Age of Divorce makes it clear that to stay married is to stick with it, to get through the "divorce periods" that Klagsbrun says all marriages encounter. Or as one longtime wife told Klagsbrun, "You just sort of keep going and know it will get better."

Married People is based on Klagsbrun's inerviews with couples married for more than 15 years, augmented by sessions spent with divorced men and women and newlyweds. Included, too, is sensible analysis by family and marital therapists (whose advice and insight Klagsbrun happily feels free to reject when her research points toward different conclusions). She spoke with blue-collar workers, professionals, easy-going husbands married to assertive wives, and hard-driving husbands married to docile mates. She probed the lives of couples who seemed to have everything in common, and men and women who were happily married although they seemed to be totally unlike each other. The results are riveting. This is a serious study, but its subjects' lives are so rich and interesting that it reads, in part, like the plot of an afternoon soap opera.

Certainly, Married People proves that happy families are not all alike. One upper- middle-class couple, for instance, had been referred to Klagsbrun by their admiring pastor, who thought their marriage was exemplary. In fact it was, but the rector of their church would never have guessed that this squeaky clean couple used to frequent Plato's Retreat-type orgies to satisfy the husband's desire for sexual variety. Another couple, loyal and supportive, had endured despite their most obvious problem: the husband's crippling lack of self-esteem. But when the stronger, more confident wife became an alcoholic 10 years into their marriage, neglecting her home and children, it was her husband who turned out to be the stalwart, holding the family together until she was willing to enter Alcoholics Anonymous. Other families' lives were less dramatic, but each of these couples had pulled through crises that might have doomed weaker marriages.

ALL THE platitudes about marriage are here, and they're all tested against Klagsbrun's observations and research. Yes, it turns out that marriage really is based on intimacy, trust, and fidelity -- but these qualitiere not present in the same proportions in different marriages. Whether these qualities were explicit or simply understood, subtle or strong, depended on the nature of each family. But all of the successful couples were accepting of each other's strengths and weaknesses; and they were able to enjoy each other's company (although none, interestingly, were advocates of constant togetherness). The other qualities that got these couples through the years were the ability to change, an ongoing interest in each other, honesty (tempered with sensitivity), and communication and compromise.

No marriage can recover from endless assault, however. Infidelity, Klagsbrun discovered again and again, is always destructive of trust, which, once broken, must be painfully won back. The ability to remain faithful despite temptation and the familiarity that the years inevitably bring, requires discipline. And discipline is, of course, hard work, requiring discretion, the ability to hold one's tongue, and a willingness to sacrifice immediate pleasure for the sake of a commitment to the marriage. In short, in order to have their marriages thrive, the partners must develop maturity and must learn to take responsibility for the effects their actions will have. As a nurse, married 17 years, put it, "Sure, I'm greedy like anyone else. I want to live in the country; I want to live in the city. I want to have a lover; I want to have a marriage. But I want the marriage more than anything. Ultimately you give up something when you have only one other person you share intimacy with, meaning your sex and your guts and yourself. But you gain more; you gain everything."

Married People is an intelligent, refreshing look at an institution that can and does thrive in half our households. And the reasons for its persistence and for the satisfaction it brings are thoughtfully and sensitively outlined here.

The couples Klagsbrun talked with have earned each other's love and compassion. They have arrived at this happy state from every possible departure point: second marriages, marriages entered into with high emotion and little sense; marriages begun reluctantly and uncertainly by partners who felt affection but little passion. They have endured the almost unbearable: the death of children, terrifying financial crises, and physical and mental illness, as well as the more mundane problems that can gnaw any relationship to death. And their tenacity is perhaps the key to these couples' fulfilling marriages. Klagsbrun is writing about individuals who choose to be with one another, who find a special grace in the closeness that the years have brought.

They aren't totally dependent on each other, however, Klagbrun wisely states: "As attached as partners in long marriages are to one another, I had the impression that they could live alone and manage if necessary because each remains an individual, wholend intact in the midst of their partnership. . . If they had to, each could survive without the other. They simply do not choose to."c