ON JUNE 18, in the Kennedy Center Concert, Washington will see something it has never seen before: a 150-voice male chorus singing George Gershwin's "The Man I Love."

Several members of the San Francisco Gay Men's Chorus have mortgaged their homes to make this possible.

The Kennedy Center appearance will be the climax of an eight-city national tour (the first in the organization's three-year history) and it is expected to run up a substantial deficit. "It we well out every performance," says the group's treasurer, Larry Geis, "that will underwrite only about half of our tour expenses, so we have had substantial fund-raising. We have about $90,000 so far, out of a projected $150,000 to $175,000. It would be a feat for any gay organization to raise that much money."

The home-mortgaging ploy became necessary because of $250,000 in advance expenses -- airline tickets, hotel accommodations and rental of halls -- that had to be paid before the ticket money started coming in. "Do we expect to cover our expenses?" Geis asks rhetorically. "We really have no alternative."

It's a speculative proposition, at best. The chorus feels secure about Lincoln, Neb., where it will be singing in a church and expenses are relatively low, but there is still some question about other cities, where the chorus is renting the most expensive hall in town because "we want to go first class all the way on this tour."

"Tickets haven't been going very well in Dallas. They're still pretty conservative there," says Hal Slate, a transplanted Washingtonian who was part-owner of the Janus and Cerberus theaters before he moved to San Francisco.

The San Francisco Gay Men's Chorus is not going to all this trouble and expense simply to show the world that some homosexual males are good singers. "I think that has been fairly well established," says Steven Prokasky, co-chairman of the tour committee. "There have been gay musical organizations for centuries, but we're the first to say so." Since the chorus came out of the closet, about 20 more overtly homosexual musical organizations have materialized across the United States, including a lesbian-gay marching band in Washington that advertises itself as "D.C.'s differnet drummers." But the San Francisco group claims to be the prototype.

"We are," says Geis, "the first gay group to stand up and say,'We're gay and we're talented and this is what we can do.'"

What they can do musically dawned on the chorus members only gradually. It began as a sort of social and recreational activity -- "an alternative to the bar scene," one member says. At first there was even some disagreement over whether everyone who applied should be admitted or prospective members should be auditioned.Now, auditions are held four times a year, and more than 80 percent of the members have previous experience with another choral group, including the choruses of the San Francisco opera and symphony, the Santa Fe Opera and the Roger Wagner Chorale. Members devote, on the average, 10 hours per week to the chorus, and San Francisco critics have applauded the results. After a concert last April, Allan Ulrich of the Examiner praised the group's "remarkable evenness, uncommon fervor and wide stylistic acumen. . . . If there exists, hereabouts, another vocal group of its size, talent and discipline," he said, "it must be hiding under a rock somewhere."

Reviewing a concert on June 5, just before the tour began, Richard Pontzions predicted in the Examiner that "a lot of concertgoers are about to have their collective socks knocked off."

And last year, in the Chronicle, Robert Commanday was positive but slightly more critical: "The tone is smooth, carefully blended, easily produced and with never a trace of strain or vocal tension. Highly trained voices do not seem in evidence, so there's no real focused power or vibrant strength." The group is interesting as a socio-cultural phenomenon, he said, but there is also "every musical reason for it to be taken seriously."

The chorus did not begin to take itself seriously -- at least in terms of such activities as a national tour -- until about a year ago, Hal Slate says. "Then, we began to realize it. At first we were not reviewed because we were thought of as a club."

Attention escalated abruptly this spring because of a public controversy. The chorus had contracted to use St. Ignatius, a large Jesuit church in San Francisco, for its April concert, which included a seldom-heard Requiem Mass by Franz Liszt. The archbishop did not learn of this arrangement until about a month before the concert, when the publicity was already out and a lot of tickets had been sold. He forced the Jesuits to cancel the contract, the chorus filed suit and tried to get an injunction, and the concert became, as Hal Slate says, "a media event." Forced to relocate to a smaller, Episcopal church (700 seats rather than 2,000), the chorus gave two performances to standing-room audiences. "We came out all right on it," says Geis.

Otherwise, the chorus seems to have good relations with the San Francisco community at large. "The whole climate out here has changed in recent years," says Slate, "and a lot of it has to do with orgainzations like ours. Our contention is that gay organizations like ours can make a positive contribution to society -- not just the gay community but the entire community. We're not going out just to play to gay people. We have learned in San Francisco that we don't have to hide, we deserve to take our place in the community and be recognized for what we are. Being allowed to function in a positive way is the breakthrough."

There is no question that the chorus is being allowed to function that way at home. At intermission of the pre-departure concert, San Francisco's mayor, Dianne Feinstein, appeared on stage with the group, declared herself "a fan of this chorus" and said she was happy that they would be "representing the cultural life of San Francisco ." The beginning of the Kennedy Center performance will be fed live to San Francisco by the Washington bureau of the local NBC affiliate, KRON-TV.

The chorus has scrupulously avoided association with political militancy, and some members think that policy has contributed to its success. "Obviously," says Slate, "we are making a political statement by including 'Gay' in the name of the chorus; we are making a statement by being what we are and doing what we do. But we don't make an explicit issue of it. The concert will conclude with the fourth movement of Randall Thompson's 'Testament of Freedom,' based on words of Thomas Jefferson. The movement begins with the words, 'The god who gave us life gave us liberty at the same time.' Singing that onstage in the Kennedy Center will be our statement.In a sense, it will be what this whole tour is all about."