Light of step but ponderous as they pose, rigid formalists who mock their own rituals, The Royal Dancers and Musicians of Bhutan practice an ancient art. All the performers are young males, but the attraction of the two sexes and the predicaments of age often form the crux of their dramatic dances. The actions can be cruel and the asides lewd, but every piece points to a moral lesson. Seldom has a Far Eastern theater of greater contrasts been seen in the West.

In its single Washington performance at the Smithsonian's Baird Hall last night, the troupe was making its d ebut outside Asia. Even there, these artists from the Himalayas have left their small kingdom only twice before. But in Bhutan their dances and playlets, as well as the accompanying texts and music, have been performed for centuries with little change.

Their vocabulary of movements is limited. Most often, the dancers bound gently in place on just one foot. Their balance is secure as the free leg is held raised in front, with the knee bent. There are also some airy dervish turns, but the really spectacular step is a high leap in which both legs are pulled up and the trunk is inclined forwrd. A final contraction into embryonic position at the height of the jump gives the performer the appearance of a rocket with second-stage propulsion.

Limited also is the musical accompaniment. There are cymbals, rattles, huge horns and, the voice.

Despite its long and constant tradition, one suspects that Bhutan's dance theater had diverse origins. The strict floor patterns and symmetry, the unisex cast and the religiosity suggest Tibetan sources. Indeed, most citizens of Bhutan are of Tibetan descent. The sensuality of the motions and dramatic themes is evocative of India. Also, these performances were seen by both the rich and the poor, which must have fostered certain contrasts, such as the sharp repartee between masters and servants.

The company's stage presence is unassuming by our standards. That may well change with more appearances outside Bhutan.