MISTY BEETHOVEN - Biograph (weekends only), K.B Studio, Shirley Duke 3.

The record for the longest continuously playing film in Washington was held, until two weeks ago, by "The Sound of Music." The new title-holder is "The Opening of Misty Beethoven," which has been playing in area theaters for 99 weeks, and is still packing them in.

What "Misty Beethoven" has that "The Sound of music" doesnt is onvious. "Misty Beethoven" is hard-core pornography. The question is what it has that is lacking in the hundreds of sex films that are in and out of town without drawing such crowds.

One hears that it has "a plot," an unusual commodity for the genre. Well, sort of. A young man who writes books about sex decides to teach a prostitute how to attract the publisher of a sex magazine. The film aims at being a dirty "Pygmalion." But it's useless to point out that the technical skills in which he drills her are unrelated to the "test," and that, indeed, the girl actually plays a very subsidiary part in the sexual scene in which she is said to have impressed the publisher. It's enough that there is a plot, without its being expected to be consistent.

But what the film does have are a touch of humor, a few mildly educated references - "Agincourt in five hours!" the teacher declares - and the trappings of middle-class preoccupations (other than sex): Self-improvement. Mental health through the ability to overcome inhibition. Success as measured by publicity: Achievement through hard work and perseverance. These are the attributes that give "Misty Beethoven" a sort of respectability, so that great numbers of well-dressed couples can attend without apparently looking over their shoulders for fear of being seen. Also, possibly because of the nature of the crowds, it has been tolerated in neighborhoods where other sexually oriented businesses are the objects of strong protests.

No doubt these trappings serve to enable people who would not walk into a peepshow store to see filmed fornication. But the relatively sophisticated film techniques also seem to recognize that sex is one of those activities which, like some sports, are fun to do but can be dull to watch with their visually monotonous basic motion.

In cruder films, sadism or other unusual acts may provide the additional interest. Here the sex is dressed up with such objects of middle-class fascination as luxurious households and cars (a great number of servents participate in the sex, which may explain why the Mercedes is so badly in need of a wash) and rules for improving one's lot in life.

Amateur psychotherapy, after all, can be just another way of playing doctor.