Ever since Dustin Hoffman bumbled his way through the mystery of cooking French toast on his first morning as a single parent in ''Kramer vs. Kramer,'' a clever little fiction has passed as fact. It has been widely and recklessly assumed that the ''sensitive'' and ''liberated'' male has arrived, and as soon as that smoke blows away from the griddle, the American family will be well in hand again. For the first time since Ibsen's Nora went slamming out the front door, a satisfactory replacement has come through the kitchen entrance: the househusband.
Once Hollywood stumbles on a cute little idea that plays well, the moguls will hang onto it as long as the box office allows. And so the myth has blown all out of proportion as we have had, in effect, ''Househusband II'' and ''Househusband III'' - and now ''Mr. Mom,'' which practically turns the French-toasting scene into a full-length movie and a thesis.
In all seriousness, what is a moviegoer to make of this latest and neatest role-reversal plot that has Dad schlepping around the supermarket, trailed by two small children, while Mom grabs for her briefcase and heads toward the executive suite?
Pseudo-seriousness is the problem. Once the jokes about runaway vacuums and overloaded washers have run their course, ''Mr. Mom'' distances itself from sitcom by implying that we are seeing social history in the Technicolor making - the evolving of the ''new man'' who will, at last, make the ''new woman'' possible.
''Mr. Mom'' cheats in several ways. As its hero mutters, ''My brain is turning to oatmeal,'' the film is clearly saying: This is what happens to people who stay home - this is what it means to be a wife and mother. And yet the househusband does become a hero - and a lovable one at that - while the wife fades away into mere ''success,'' ever more shadowy and coolly detached, while a subliminal voice on the sound track seems to say: ''See? This is what happens to women who work.''
But the worst sleight-of-heart involves a second sneaky role reversal that has Mr. Mom ending up in the workplace again - traditional Mr. Dad - while Nora winds up back in the doll's house.
What are the facts, as contrasted to this 1980s update of ''Ozzie and Harriet''? First, the househusband is a token male to end all tokens. In a recent major study of American couples, sociologists Philip Blumstein and Pepper Schwartz found only four husbands out of over 3,600 who said they care for the house full time. ''Married men's aversion to housework is so intense it can sour their relationship,'' the researchers write. ''If this pattern continues into the future, it will be a major barrier to the reorganization of husbands' and wives' roles.''
Even in two-career households, according to survey after recent survey, men simply are not doing what could logically be considered their fair share of housework. Real men may eat quiche, but they sure don't dust.
''It's not so easy, is it?'' the working wife asks her husband in ''Mr. Mom, '' speaking of a homemaker's lot. ''It might even be the hardest job in the world.''
Finally we're getting close to the truth. Now it's time for someone to present it that way - with humor, please, but also with the kind of honest sensitivity we have come to expect from all these ''sensitive'' male heroes. If we're willing to cheer when men assume these roles, why can't we stop snickering when women perform them?
Perhaps it's also time for Hollywood to consider another kind of role reversal - one far less chic but much more prevalent. Instead of idealizing fathers who manage to keep a household running, movie studios could produce a story about mothers who do the same - with no help from a husband. Millions of divorced mothers could offer advice on the script - women who do not have the luxury of participating in decisions about who will work and who will take care of the children. For them there is only one answer: They will - alone.
In any case, it's time for all of us to stop treating the infinitely varied duties attendant to making a house and rearing children as a second-class, if not third-class, achievement.
Can ''nurturing'' be given a good name? It had better be. As both parents tend to get busier and busier, the needs of the family are becoming more complex than ever before, and the fulfillment of these needs depends on a lot of little things. Somebody had better want to do the jobs we have so successfully defined as ''unglamorous.'' And somebody had better want to do them well.