WITH all the recent talk about obscenity in song lyrics, there may be something to learn from ''West Side Story,'' the beautiful musical drama that opened The Muny season Monday night.

''West Side Story'' debuted in 1957, still very much the era of the euphemism. The lyricist, Stephen Sondheim, probably did not consider even for a moment whether or not the young street toughs whose tragic lives unfold should talk the way young street toughs really talk - yes, even back in the Dark Ages of the Eisenhower years.

Instead, he had them sing derisively to an imaginary, detested police officer, ''Gee, Officer Krupke, krup you!'' He had them claim ownership of the ''whole buggin' motherlovin' street.''

Was it authentic? No. Did it work? Oh yes - and it still does.

In part, Sondheim's creativity makes the inventive curses more effective, because they are unexpected.

But it also means that the strongest words in the show - the ones that do slap you in the face - are the racial epithets that the gang members hurl at each other: ''mick,'' ''polack,'' ''spic.'' They sound ugly and raw here, as they should. And, in that brutality, they bear the burden of the play that is not really about street life, not in any sociological sense, but about love and prejudice and what harm one can do to the other.

The Muny gets off to a fine start this year with a production that is true to the extraordinary spirit of this work, the creation of theater geniuses including Sondheim, composer Leonard Bernstein, director-choreographer Jerome Robbins and, last but not least, William Shakespeare. It was Robbins who first thought of setting ''Romeo and Juliet'' in the slums of New York, recasting the star-crossed lovers as Tony, a longtime member of the Jets, now trying to lead a better life, and Maria, an innocent Puerto Rican girl whose brother, Bernardo, leads a rival gang, the Sharks.

Alan Johnson has an unenviable program credit explaining that he ''reproduced'' Robbins' direction and choreography. It sounds kind of mechanical. But so much of the emotional weight of the show - so much of the story itself - is carried by the dancing that to change it would be, essentially, to destroy ''West Side Story'' and replace it with something else.

Johnson has, in fact, brought together a wonderful group of dancers and obviously demanded a lot of them. From the steamy mambo at the gym, to the virile gang fight, to the idyllic ballet in which the lovers imagine a different, tolerant world, the dancers' range is impressive not just physically but emotionally.

The stylized dancing of the gangs has, of course, been joked about - if only gangs just danced! But here, led by Robert Montano as Bernardo and Michael Gruber as the Jets' leader, Riff, it takes on a reality of its own.

The dancing isn't meant to be true-to-life; it's meant to convey the bravado, the self-destructiveness and also the beauty of kids with nowhere to go. That it does. And Jackie Lowe, who plays Bernardo's girl friend, Anita, gives a passionate performance that is just about perfect.

Just a little of that passion wouldn't have hurt Peter Gantenbein and Betsy True, who play Tony and Maria. They are both very talented as dancers and as singers, and they even look right. But sometimes it seemed as though they were so afraid of overplaying these most emotional of roles that they held back a bit too much.

But when they sing, it hardly matters; all the tragedy, all the beauty is there in their voices. This wonderful score includes ''Tonight,'' ''Maria'' ''Somewhere'' and ''America.''

Before the show started, The Muny's new executive producer, Paul Blake, took the stage to welcome the audience, thank the theater's supporters and remind people that tickets were still available for the rest of the ''West Side Story'' run. He suggested that if they enjoyed the show, they tell their friends. No doubt many will.