I WONDER why I am so reluctant to say mean things about Erich Segal's new book.

Maybe it's because people whose opinions I respect seem to like the book. Or, maybe it's because I have a fellow-feeling for an academic type who sold his artistic soul to the devil of commercial fiction in exchange for some jam to spread on his dry academic bread. It certainly can't be because I am afraid I will hurt Segal's feelings, or because I am such a nice person that I hate to criticize other writers.

The protagonist, Bob Beckwith, is an MIT professor. His wife is an editor. They have two young girl children. All of them are nice -- attractive, intelligent, sensitive. The relationships of husband and wife, parents and children, are well-nigh perfect.

As the book begins we learn that Bob hasn't always been so nice. Once, 10 years earlier, he had a brief love affair with a liberated, attractive French doctor. Bob has almost managed to forget this unforunate episode when he gets a telephone call informing him that his former lover is dead, and that she has left a child -- his son. After breaking the news to his hitherto unsuspecting wife, Bob agrees to take young Jean-Claude for a month while friends in France try to make arrangements for his future.

Now I am willing to believe that there are wives, even in this cynical, permissive society, who would react with violent shock and hurt to a man's admission of one brief, bitterly regretted episode of physical infidelity 10 years in the past. I am not sure I am willing to believe that her shock would take the form of a mid-Victorian blend of cold rejection and repellent nobility -- for Sheila not only agrees to take the child, she suggests it herself. And I seriously doubt that a man would humbly accept weeks of such punishment without getting roaring drunk or throwing something, at least once.

Bob does accept it. The boy arrives. He is just as nice as everybody else in the book, if not nicer. Naturally no one except Bob and Sheila knows his true identity, but before long both of them yield to the urge to share their troubles and each confesses the truth to a close friend. Bob's friend tells his wife, and their discussion is overheard by their son, who blurts out the news to Beckwith's daughters. A violent family confrontation ensues. Jean-Claude leanrs that Bob is his father. Sheila tells Bob he must send the boy back to France that very night, or lose his family.

Bob starts out for the airport but finds himself unable to part with his son so abruptly. They spend a few days together. Jean-Claude develops peritonitis and almost dies. During his convalescence Sheila and the girls have a change of heart. They are now prepared to accept the lad as one of the family. Bob, of course, has learned to love his son and Jean-Claude obviously adores his newfound father.

Happy ending, right? Wrong. I am tempted to give away the ending, but out of courtesty to a fellow romancer, I will reluctantly refrain. Be prepared, however, to wring out several sodden hankies.

The word "contrived" has been used to describe Segal's plots. The word is accurate, but its pejorative implication is not; all plots are contrived, in the sense that the author, having determined what the denouement is to be, invents incidents designed to make that denouement believable. Segal's problem is that his plot is not contrived enough. The pathetic ending does not logically result from anything that has preceded it. It is not unexpected -- Segal's readers know they can expect a good cry somewhere along the line -- but it is unfair. There is nothing wrong with waving an onion under a reader's nose in order to induce tears, but the reader is entitled to be annoyed when he realizes that the author is trying to wring his heart with a rutabaga or a stalk of celery. The failure is all the more annoying in this case because it would have been just as easy to use an onion. A situation like the one Segal has invented is absolutely replete with elements which could reasonably bring about a soggy, sad, pathetic ending. Sheila's resentment and jealousy, the inability of the two young girls to cope with a permanent relationship with a bastard half brother, Jean-Claude's own resentments and doubts -- these, and many other obvious factors could prevent the pretty family reunion the reader has been led to expect. Segal raises all these nasty specters only to blow them blandly away.

It would be illogical to criticize Segal for his refusal to develop his characters into real people with unresolvable problems; one might as well criticize him for not writing Hamlet or Anna Karenina. However, he is liable to the charge of shoddy craftsmanship when a neat workmanlike job would have been just as easy.

Few readers will complain of this, I fancy. My dignified great aunt used to say that she liked "nice books about nice people." Allowing for the inevitable changes in mores and speech patterns since her day, that is what Man, Woman and Child is -- a nice book about nice people.