Minimally sugar-coated and cuted up-by Hollywood television standards, that is-"Like Normal People," the record of a retarded couple's struggle for acceptance, makes a moving and enlightening two-hour ABC movie, tonight at 9 on Channel 7.

The true story, from a book that began as a series of articles in The Washington Post by staff writer Robert Meyers, deserves a far more detailed and attentive treatment than it got; as film-making, the production is patently undistinguished. But the strength of the reality behind it, the affirmative essence of the material, and a few diligent performances give "Like Normal People" a core of rare dignity and an undeniable emotional impact.

The Meyers pieces, first published in 1977, were remarkable for their unaffected candor and the new immediacy they brought to the subject of mental retardation. Suddenly we were made aware of how little we knew, how much we had relied on a foul kind of folklore. Meyers used the biography of his brother, Roger, for the focus, but put Roger's story in the context of traditional attitudes toward the retarded.

In the TV script by producer Joanna Lee, the social context is largely absent or trivialized; it is personified mainly by a few little boys who taunt young Roger at summer camp and, later by a bigoted hospital administrator who opposes the marriage of two retarded people, Roger and Virginia Rae Hensler. "They must not reproduce," the man thunders.

Despite simplifications and streamlining, the firm still illuminates innumerable facets of this situation and of all human prejudice and handicap. If a happy ending is virtually always in view, "Like Normal People" at least dares to be fitfully painful and troubling.

The casting of teen idol Shaun Cassidy as Roger may sound shallow or cynical. In fact, Cassidy works so hard and earnestly that not only is disbelief suspended, but you find yourself as drawn to Cassidy's struggle as to Roger's, and in a way, they become the same. It is true he occasionally sounds as if he is mimicking Jimmy Stewart's drawling delivery in his effort to "sound" retarded, but at crucial moments, as when he first hears the word retarded applied to himself and weeps in his mother's arms. Cassidy is convincing and admirable.

Linda Purl, more accomplished in acting, is surprisingly inadequate as Virginia, whom Roger meets near a stuffed rabbit in a residential facility. Both actors studied tape recordings of the real Roger and Virginia speaking, but it Purl's version is faithful it is dramatically counter productive.

As Roger's mother, however, Hope Lange brings a bracing conviction to the kinds of problems and frustrations a child such as Roger can add to a household.

And Zalman King, called upon to embody 30 years' worth of progress in the way society treats retarded people, nevertheless makes strong and believable the character of Bill Stein, who works at the residential facility and takes an interest in Roger and Virginia's plight.

The real Roger and Virginia can be seen late in the film, as extras at a party. They are sitting on a couch and laughing with other guests.

It is hard to ascertain if the director of the film, Harvey Hart, is a strict minimalist or a clod, based on his previous work, the latter is more likely. The film is utterly flat and graceless but then, on the other hand, this may be preferable to gross sentimentalization, an alternative so many in Hollywood would find impossible to resist in a case like this.

That the film's original and yechy title-"A Very Special Love"-was thrown out says something for the integrity of the filmmakers. Though when we first see them, Cassidy and Purl look overly adorable - almost like kids trying to imitate Laurel and Hardy. They slowly and persuasively become real and genuine. They move in next door, and we are grateful to have known them. CAPTION: Picture 1, Linda Purl and Shaun Cassidy in "Like Normal People"; Picture 2, Shaun Cassidy and Linda Purl, background, with Roger and Virginia Meyers, whom they portray in "Like Normal People."