For a while, "Gay in Washington," a WETA-produced documentary at 10:30 tonight on Channel 26, looks as though it will accomplish the seemingly impossible: discuss homosexuality for half an hour without once mentioning AIDS, suddenly the most talked-about disease in the Western world. But then, about two-thirds of the way into the program, reporter and producer Sheila Banks adopts a very grim tone and says that "a new dark cloud is rapidly spreading" through homosexual citizenry, and the program's single hope at distinction evaporates.

At the center of the program--generally, a weak and inferior piece of work--there is the potential for a revealing study, perhaps to be called "Black and Gay." Banks is talking with members of various black homosexual and lesbian groups and they tell her that there is still considerable discrimination against, and harassment of, blacks in District establishments, especially bars, that cater to gays.

This is happening in a city that is 70 percent black, and the racism is being practiced by members of a social class who surely know at least vaguely what it's like to be the subject of intolerance. The topic may be difficult to deal with, but "Gay in Washington" chooses the easy way--by bringing it up and then dropping it for a segment on AIDS that seems to assume no one has ever heard of it before.

At first the report deals with growing gay political clout. Marion Barry is acclaimed by one activist as "the most pro-gay big-city mayor in the country." Sen. Alan Cranston (D-Calif.) is seen courting gay votes. Then the rhetoric is mercifully abandoned for an interview with Lilli Vincenz, a pioneering gay activist who says she has been picketing and protesting since 1963. It is hard to imagine even the most unsympathetic viewer encountering the articulate Vincenz and not seeing courage there.

Among those in the group of gay blacks who talk to Banks is a man identified as "Joseph" who says he has a 6-year-old daughter whose grandmother told her that her father was "a faggot." And there is "Conte" (pronounced Kon-tay), a transvestite who wears large pyramid earrings and says, "This is my statement . . . dressing in female attire."

Then it's on to AIDS and plenty of old information. Perhaps the best and least hysterical discussion of the subject so far on television was the one that led off Sunday night's syndicated "Larry King Show" on Channel 9; King's approach was impeccably level-headed. By comparison, the primer that ends "Gay in Washington" is a mere digression, and a digression from a program that never does decide what its central focus is.