TWENTY GENERATIONS of Phantomas have defeated pirates, assassins and the great variety of other villains who dare to oppose comic strip superheroes. But the current Phantom has met his match in more ways than one.

He has just become engaged to marry a feminist.

The Phantom was in bad enough emotional shape when, last month, he finally worked up the nerve to propose to Diana Palmer, whom he had been courting for the last 20 years. But when Diana accepted him and then went on to inform him that she had no intention of living with him merely as his Phantom-consort, that she had her own career as a United Nations administrator, and that she planned to commute to New York, rather than stay in his Skull Cave in the Afro-Asian country of Bengali, he was visibly shaken.

"But . . ." he stuttered, "my wife must live with me . . ."

"Dear", replied Diana kindly, "I know all the Phantoms' wives for 20 generations stayed in the Skull Cave. But this is 1977."

So it is, and the Phantom had better get used to it. Lee Falk, author of the comic strip - which started in 1936, is drawn by Sy Barry and is now read by 100,000,000 people in 40 languages - says we can expect a wedding in the next few months.

"There are going to be a lot of things the Phantom is going to have to adjust to," he said.

It is this difference in lifestyles which has kept the couple, who met as children, from marrying all these years.

The Phantom is the heir to a title founded when one of his ancestors, the son of Christopher Columbus' cabin boy, took an oath to avenge his father's death at the hands of pirates. On the skull of the murderer, he vowed that he and his sons after him would devote their lives "to the destruction of piracy, cruelty, injustice."

Interestingly enough, this is the same objective which Diana Plamer has in working for the United Nations Human Rights Commission.

But there's wide difference in the way they go about it.

The Phantom sleeps on the floor of his cave, eats berries and uses his vast wealth - jewels given to generations of Phantoms by grateful potentates - for travel expenses.

Diana comes from a wealthy background and has a taste for opera, ballet and society. She does not sleep on the floor. "Maybe the Phantom could have wall-to-wall carpeting installed in the cave," Falk said as a possible compromise. He also believed she might like to have "indoor plumbing installed there - I don't think there is any."

But she plans to take an apartment in Manhattan - "probably near me; I rather fancy Diana," said Falk - to be near her job and although she may eventually have the occasion to tak maternity leave, the only circumstances under which Falk envisions her leaving her job would be "the offer of a better one".

The Phantom actually has three hereditary residences, and many priceless treasures which should delight a cultivated woman.

In the Skull Cave, he has a Minor Treasure Room, filled with those inherited jewels from which he will soon select a diamond for his fiancee, and a Major Treasure Room, insured only "by his own fist", in which he keeps such priceless objects as Homer's lyre, the swords of Roland and King Arther, Cleopatra's asp and the golden laurel crown of Caesar. The pride of the collection is a diamond drinking mug owned, in turn, by Alexander the Great, Caesar and Charlemagne - on which all of them scratched their names.

He also has a castle in Europe, now in ruins, but possibly restorable as an appropriate residence for Diana. At the moment, the Phantom occupies the cellar when the visits it. And he has a place among the mesas of the Southwestern United States.

He was educated in the United States. The Phantoms have married women of many nationalities - including a French queen and a Swedish barmaid - and the tradition is that the son goes to school in the country of his mother. This Phantom's mother being an American, he was sent, at the age of 10, to his aunt in Mississippi. That's where he first met Diana, who is from Westchester, N.Y.("or maybe Chevy Chase, I don't think I ever quite said." the author noted) but went to boarding school in the South.

Just before his college graduation, on the eve of a celebration honoring his athletic triumphs, the Phantom, who then went by his civilian name of Kit Walker (for "The Ghost Who Walks") was called home to his father's death-bed and the title.

He then has to don permanently the Phantom costume and mask. This was adopted so that the succession of different-looking Phantoms would all look alike, preserving the popular but erroneous notion that the Phantom is immortal.

Meanwhile, Diana was making the social circuit.From time to time, the Phantom would visit her while in the United States on business, and although her mother and uncle dislike him and have hoped for years that she would marry better, a room of their huge house is furnished only with straw mats and set aside for him.

"Her mother has a hard time explaining that room to her friends", said Falk. It is also hard to explain what happens on those visits. A window is always left open for the Phantom, who appears secretly to see Diana in private. Falk was asked if he meant to say that there has been an affair going on here for 20 years. "I didn't say that," he replied. He was then asked if he meant to say that the Phantom and Diana were up there secretly doing nothing all these years. "I didn't say that."

At any rate, Diana tired of the social rounds, became a nurse and joined the United Nations Worl Health Orgainization. On various expeditions, she would run into the Phantom, who was plying his trade around the world, and once she was even sent to Bengalli, on a medical team headed by Dr. Lamanda Luaga, a former Rhodes Scholar who is now President of Bengalli and will serve as best man at the Palmer-Walker wedding.

Diana transferred from field work to administrative duties at WHO, and moved up rapidly. She will carry a high rank in her new job at the Human Rights Commission.

She does neet to Produce an heir. Although the Phantom has adopted a founding, the Phantomship must descend in blood line. Males take precedence, but if Diana does not have a son, her eldest daughter will become the Phantom. There is a precedent for this - a woman became Phantom several generations back, when he twin brother was murdered - and recently Diana discovered the female Phantom costume in the cave and perhaps somewhat symbolically, tried it on.

At any rate, the entire relationship is a dramatic contrast to that of Brenda Starr and her husband, Basil St. John, in the comic strip bearing her name. During the decades of their courtship, Starr used to admit that she only worked at her job because she was disappointed in love, and when she finally did get married, she momentarily succumbed to her husband's desire that she quit her job, bore him no rancor for plotting to get her pregnant without her consent, and now, after all those years as a star reporter, does not seem capable of doing her job without having him there literally to hold her hand on assignment.

it is hard to imagine Diana standing still for such treatment. As she told the Phantom, "But this is 1977."

Incidentally, Falk himself started off 1977 by getting married. His wife, Elizabeth Moxley, was for many years a cosmetics executive - she named an Alexandra de Markoff lipstick Mandrake Red after Mandrake the Magician, which Falk also writes. She is now writing what her husband describes as "the most horrible horror novel in history."

He also credits his marriage to her with the Phantom's sudden very strong desire to get married.