It is no secret that painter Pablo Picasso, until the end of his days, loved women and considered sex a force of nature. It is explicit and implicit in his works. But not until Barcelona's Museu Picasso put on its "Picasso Erotic" exhibit has the great Spanish painter's celebration of the male and female organs been shown in the perspective of his youth, his middle-age, and his old-age.

What's startling about the exposition of 58 erotic works are 11 drawings never shown publicly before. Picasso did them in Barcelona at the turn of the century when the budding artist, then in his blue period, was cutting a swath through the swinging Catalan city's artistic and night life.

They stand out because they show Picasso as a voyeur in a Barcelona bordello portraying the dead-pan faces of his friends-who are fully dressed-being serviced by the donas . They are, in a way, rather sad, particularly a self-portrait entitled "Nude With Picasso Seated at Her Feet."

These works, kept out of sight for fear of censors and vandals during the Franco dictatorship in Spain, arouse all kinds of comments from the hundreds of visitors who climb three flights of stairs daily to view the display of erotica-called Picasso porno by the Spanish newsweekly Cambio 16.

"They are phallic and little corrupt," said architecture student Theresa Power, 20, of Duluth, Minn. "In the early stuff he's a peeping Tom, then he's a participant, then, when he's older, he's a peeping Tom on himself." But Power and a friend, Denise Haraden, 20, of Avon, Ohio, liked the exhibit because "we can see how he could see himself."

Pierre Gibert, 20, of Beziers, France, was disappointed. "He's obsessed with sex," Gibert said. "It lacks love." His friend, Ann Marc, 17, found it "Crude." A Swidd couple, Nicole Bezencon, 20, and Lubor Borkovec, 23, literature students, differed in their reaction, too. "It's the third time I've seen it, and it shows that he was a great human being." She wasn't so impressed. "It's funny," she said.

"How silly to paint that," remarked Alberto Abreau, 15, standing with a group of Catalan teen-age boys and girls in front of a drawing of a dona on her knees practicing a sexual act on Picasso's painter friend Isidre Nonell, who sits looking impassive in coat and tie.

The group was amused, too, by another bordello scene in which Picasso, in a style vaguely reminiscent of Toulouse-Latrec, portrays another joyless friend, Angel Fernandez de Soto, fully dressed and smoking a pipe, and a dona seated on his lap with a raised champagne glass, sexually massaging each other.

Catalans of all ages have a grand time with Picasso's funny collages of pinups of the 1950s, such as Esther Williams and Lana Turner, on which he drew his friend Jaume Sabartes as a lecher and wrote a note in color pencil urging him to get busy and make love. The exhibit's publicity poster is one of these collages, done in 1957, with a photograph of Picasso looking down.

Young people in jeans, sweaters and parkas outnumber the middle-aged and the old. The young troop in alone and in groups and appear to have a good time. They are easy about it, and express their reactions without inhibition. The middle-aged and the old-no doubt still thinking of the Franco era-are uptight and tend to clam up.

Felipe Ruiz, 56, a bank manager, said, "It's all right, but I don't like pornograph. The erotica is fine. But I think that the exhibit demeans the man who painted Guernica, that great portrait of criticism of Franco's outrage against the Spanish people. I like to think of Picasso that way, and not in a whorehouse."

So far more than 20,000 have seen the show. When it first opened, late last month, guards tried to stop teen-agers from entering. The show was so popular its run was extended and it will shortly travel to Paris and, in the fall, to Florence.

"The exhibition is really an entertainment," said critic Mercedes Laso. "But it brings attention to the museum and to Picasso. He has more significant erotica-like the minotaur. For me, he tossed off most of these works to pass the time. He was always busy, always creating."

There is no doubt that the display has focused attention on the museum-founded in 1963 to collaboration between Picasso, his friend Sabartes, and powerful Catalan politicians in Barcelona's city hall. Franco was in the heyday of his powers, and Picasso, an exile in France who refused to set foot on Spanish soil while the dictator lived, was among the regime's greatest enemies. But Franco did nothing to stop the installation of the museum, with Barcelona funds, in a huge mansion in the city's gothic quarter.The museum has no great Picasso works, but what it owns and shows is Picasso's early development as an artist in Barcelona, Spain's most stimulating city. Franco hoped that Picasso would sue for peace and give his native Spain important works-including Guernica, Picasso's condemnation of the bombing of the Basque shrine in the 1936-39 civil war.

Picasso loved Barcelona, and Barcelona in turn claimed the Andalusian painter, whose activity began to emerge in the Catalan capital's swinging Bohemia between 1900 and 1903. He enjoyed the bordellos, the cafes, and fencing with his umbrella the trees on the Ramblas, those wide Barcelona avenues, after drinking and talking bouts with writers and artists. The love affair between the artist and city was enhanced by his close friendship with Sabartes, a Catalan. Many of the works in the museum were given by Picasso to Sabartes, who in turn donated them to the museum. The erotic collection, except for the early porno, a gift from Catalan banker Luis Garriga Rioq, all came from Sabartes.

The city council's cultural delegate, Joan de Sagarra, promoted and was responsible for bringing the works out of the museum's cellar and putting them on display. He explained that "in Franco's days censorship was imposed by officials, so the museum, rather than risk problems, censored itself." Sagarra has been criticized for sponsoring the exhibit, but museum officials deny published reports that at least three works were defaced by rightist vandals. If this happened, they have been expertly restored and put back on show.

"It just didn't happen," said museum official Madalena Guell. "A few people are just trying to turn an art exhibition into a political or a moral issue, trading on Picasso's name and fame, but they have failed."

For art critic Luis Gelabert, 45, Picasso was a symbol of resistance against Franco, and, "Now the newspapers and magazines have taken one facet of his work, and turned him into a pornographer. That only convinces the right, and the conservatives, that he was naughty because he was an artist and Communist. But see how the city's power elite is now using him to show they are liberals and democrats? It's true that Picasso was up to his navel in politics, but that didn't interfere with his art. I think, for instance, that some of these works should be part of the museum's permanent exhibition. They shouldn't go back to the cellar."

Apart from the early 1900s Barcelona drawings and the pinups, most of the other erotica has been shown before. What's fascinating is the progress from the detached peeping-Tom when Picasso was in his 20s to the lust of a series of 10 prints, called "The Embrace," which depict, in various styles, a couple entwined in the act of love. These prints-all in one room-put Picasso in the role of participant. He is painting himself as a lover.

The final part of the exhibit-the envoi-is something else. Done in 1968, the 21 etchings include the painter in the drawings and he is a voyeur on the scene complete with palette and brushes time and time again.

"It's clear that when he did this last series Picasso was beginning to feel impotent as a lover because of age," said critic Antonio Garachena. "It's the sequence that makes the exhibit. I think the museum has been very smart in the way they have put up the works. You go from shock and amusement to passion, and end up with detachment, but still he recalls the fire that is lacking in the early material."