Joe Haggerty goes to dirty movies a lot. He sits in pornographic movie houses and writes on a yellow legal pad what naked couples say and do on the screen.

Judges say what Haggerty writes is obscene and attorneys joke about his graphic prose. But writing dirty is what Haggerty is paid to do, and he's a pro. Haggerty is an investigator in the D.C. police department's morals division--the one law enforcement officer in the city assigned full time to rooting out obscenity in Washington's movie theaters.

The lurid descriptions he writes are submitted as affidavits to prosecutors, who lately have been giving them to judges in D.C. Superior Court. The judges, in turn, have ordered some of the films seized.

In the last year, seven of the films Haggerty wrote about have been confiscated from theaters for violating the city's obscenity laws--more films than in the previous four years combined. The most recent, "Desire for Men," was seized two weeks ago at the D.C. Playhouse on 15th Street NW, not far from the White House. Others awaiting prosecution are "Prisoner of Pleasure," "Silky," "China de Sade," "Dominatrix Without Mercy," "The Satisfiers of Alpha Blue" and "The Analyst."

Police took the films--all from theaters in the downtown entertainment district--to the courthouse, and that's where they are today. Nobody's quite sure what to do with them, because nobody has been quite sure how to enforce the obscenity laws, which forbid the display of obscene materials but do not specify just what is obscene.

In recent years, prosecutors generally have not been keen on bringing obscenity cases to court in Washington. The work is slow and cumbersome, and few cases result in convictions or substantial penalties. As long as there are rapes and robberies on the court docket, prosecutors say, obscenity must wait.

So why are police confiscating so many films? Mostly, according to prosecutors, because of Haggerty.

Haggerty worked as a detective in 3rd District's prostitution unit, but he says the hours were long and irregular, his personal life strained. Two years ago, at 34, he asked to be transferred to the morals division at headquarters downtown.

"I got married," he says, his shoulder-length, prematurely gray hair combed straight back. "I needed to spend more time at home."

The transfer, says Haggerty, made life easier at his suburban Maryland home. His new wife "survived me working prostitution when we were dating. She accepts it all right." But he says the new job brought problems of a different kind.

More than four years have gone by since the last time prosecutors showed pornographic films to a jury in the city. As a result, prosecutors have no "community standards"--the benchmark established by the U. S. Supreme Court--on which to judge what is obscene. Since September 1980, when he took the job, Haggerty has been enforcing what he calls "general grossness": bestiality, violence, defecation, urination, child pornography and anything else that strikes him as lurid or "unnecessary."

Haggerty says he does not consider himself a "censor." Yet few people complain about the films Haggerty asks to be seized. "To some degree, like prostitution, it's just bringing some of this stuff to the attention of the community," he says. "If half the general public knew what goes on in the pornography industry, they would scream bloody murder."

Historically, District juries have been lenient toward sexual promiscuity in films. Prosecutors say that is one reason they have been wary of taking obscenity cases to court. In January 1978, they lost what was considered a major case against Mickey Zaffarano, former owner of the D.C. Playhouse.

In 1978 police presented affidavits against 43 films, and prosecutors authorized one film seized. In 1979 police presented 26 affidavits and one film was seized. In 1980, 27 affidavits were presented and two films were seized.

The recent increase in confiscations of films--Haggerty's record of six films in six months--stems in part from Haggerty's zeal, and in part, prosecutors say, from a new policy of referring such cases to D. C. Superior Court instead of the federal U. S. District Court.

Haggerty recommended that "Desire for Men" be confiscated because of a 15-minute scene at the end of the film. It involved a sexual act between two women, one of whom as an amputee.

Prosecutors at federal court likely would have overlooked what Haggerty found to be "gross." Over several months last year he had numerous discussions with officials there about the lack of interest and manpower in enforcing the obscenity law. Assistant U.S. Attorney Donald E. Campbell, head of major crimes for the office, agreed to shift obscenity to Superior Court, a move he described as "meat pushing."

"When you have to make the decision about what are the most important things to spend your time on--in that sense, yeah, we were glad to get rid of 'em," Campbell said.

In Superior Court, Haggerty's affidavits so far have received the kind of attention he was hoping for. Prosecutors there, accustomed to a daily diet of more mundane offenses, say they are delighted with the chance to delve into the complexities of an obscenity case.

Still, authorities say there is no guarantee that the films confiscated recently will be prosecuted, and are doubtful that the string of seizures in the last six months--along with the arrest and subsequent release of two theater owners last March--will continue long into the future.

Prosecutors are considering showing several of the films to a grand jury "to get a reading" of what District residents consider obscene. But as long as theater owners keep the films in the downtown tenderloin district, and away from residential areas, juries, they said, will probably find no harm in the movies being shown.

Haggerty's job for now, the prosecutors say, is to keep the owners in line.

"When I was in prostitution, I always wondered if there was a job anywhere with a lower priority," says Haggerty with a world-weary grin. "Well, I think I finally found it."