The unacceptable outcome of El Salvador's civil war perched uncomfortably on a leather-covered couch in the quiet hillside villa, sipping Coca-Cola and arguing that Alexander M. Haig Jr. and other Americans had nothing to fear from him and his Marxist guerrilla comrades.

"Yes, the high command is Marxist," said Eduardo Sancho Colombari, who, under the nom de guerre of Ferman Cienfuegos is one of the five senior commanders of the guerrillas in El Salvador. "But it is a Marxism that is 100 percent Salvadoran...we know we have to act with great realism and seek a policy of coexistence between our little peoples of Central America and the United States."

The Reagan administration has defined its primary objective in the Salvadoran conflict as preventing Cienfuegos and leaders of four other Marxist groups in the Farabundo Marti National Liberation Front military command from coming to power because of their support from Havana and Moscow, their history of violent subversion and their ideological commitment to communism.

Throughout a two-hour interview, Cienfuegos, 34, a former teacher of art history, repeatedly sought to allay American fears that an El Salvador run by the guerrillas would become, in his words, "a satellite of Cuba and the Soviet Union. We know of this concern, but it is not the real situation."

Two years of armed warfare by his Armed Forces of National Resistance and the four other guerrilla groups against the military-civilian junta in neighboring El Salvador have produced "a unified conclusion" among the guerrillas that a coalition government, a mixed economy and a nonaligned policy that does not threaten the United States are the best way out for their war-devastated nation, he said.

"We have reached agreement on a political program to follow at this historical stage. It may not be born from one's wishes, but it is the result of our analysis of the war. It is the Salvadoran people who are demanding unity, and we represent our people. There is also a change in the world situation, and we must respond."

Cienfuegos did not elaborate on the nature of the change, but his other comments suggested that concern by Nicaragua and Cuba over the Reagan administration's deepening military involvement in the region was spurring the guerrillas to seek negotiations--while continuing their fighting--with President Jose Napoleon Duarte and his high command without any prior conditions.

A muscular man with a heavy black mustache and several days growth of beard, Cienfuegos spoke in matter-of-fact tones about the escalating war and the growing possibility of "an American intervention--a madness, a bit of adventurism," but he grew more voluble when pressed to explain why Americans should believe that the guerrillas will not follow Fidel Castro's pattern of becoming a strategic ally for the Soviet Union once in power.

"I have been to Havana twice. I met with Fidel on one of those trips. But I have also met Mexicans, Nicaraguans and Venezuelans, and would like to have better relations with Americans. It is not true that we receive arms or orders from Cuba or the Soviet Union," he said.

Cienfuegos' guerrilla group "made itself famous by capturing businessmen" in the mid-1970s and holding them for ransom "so we could build up a national war chest. Because, when we began the war, we never thought we could depend on Cuba or the Soviet Union. Our people are in this war because of a dictatorship imposed on us for 50 years."

R. Bruce McColm, in a recent booklet published by Freedom House, estimates that by 1979 the Armed Forces of National Resistance had amassed $60 million through kidnaping and extortion. That sum would buy the guerrillas as much in military equipment as the Reagan administration is now sending in emergency aid.

This guerrilla group is considered by some experts to be perhaps the least doctrinaire in the guerrilla high command, and Cienfuegos used little Marxist jargon in his descriptions of the war. But the two hours of conversation revolved around what essentially was a request from Cienfuegos to Americans skeptical about the guerrillas that could be summarized as, "Trust us."

Cienfuegos said the guerrillas had overcome deep divisions, which had caused them to delay their first all-out general offensive from May 1980 to January 1981. Asked about the 1975 assassination of a comrade, Roque Dalton, a leader within the People's Revolutionary Army, he said that it was a "historical accident that has been overcome." Dalton's assassination reportedly was the cause for Cienfuegos' group splitting away from the older People's Revolutionary Army.

The interview, arranged through Nicaraguan officials, appeared to underscore a determination by the guerrilla leadership to show that they support and are bound by the political positions that have been put forward abroad by their non-Marxist political and diplomatic spokesmen who are grouped in the civilian Democratic Revolutionary Front.

The five guerrilla leaders in January sent a letter to President Reagan stressing their commitment to negotiations, a reversal of their previous refusal to talk to Duarte.

Future relations between a coalition government including guerrillas and the United States "will depend on what the United States can do, and what we can do," Cienfuegos said. "Our revolution would not be a danger for the United States. Our people would receive aid for our development from all the peoples of the world, but we would not be part of any hegemony from any power. If the United States tries to help our people, our people will respond. If the United States tries to drown our people, we would look for help from the other side.

"We are realists. We use Marxist analysis, but so do professors at Boston University and other American universities."

When Cienfuegos observed that a superpower with nuclear weapons could not reasonably fear a smaller, military weak neighbor, he was asked about the Soviet Union's reaction to events in Poland.

"We are not in agreement with any intervention of the Soviet Union in Poland," he replied. "The Polish people should have self-determination."