In Rod Decker's Channel 2 "Take 2" call-in discussion program a week ago Sunday, the first caller asked why Decker had chosen that night's topic. Was it to hype the ratings during the critically important February sweeps?

It was a fair question, and one a person as perceptive as Decker certainly knew would come up. Decker's guests were two therapists who were discussing sexual dysfunction in marriage. They weren't being a bit prudish about answering Decker's questions about female orgasm, oral and manual sex and masturbation.Furthermore, Channel 2 had teased the program generously on air and had warned that the subject matter was mature, almost a guarantee of getting an audience.

Perhaps most suspiciously, sex topics are a staple of a whole new genre of television programming that has erupted in the networks' mad race for a larger slice of the declining TV audience. The programs include the quasi-news discussion programs. These are much watched though also much criticized as "sleaze journalism," "tabloid TV" and, along with the increasing number of dramas of lust and passion, "trash TV."

The long-reigning exponent is Phil Donohue. He recently has interviewed transvestites and male homosexuals, among other sexual aberrants. Geraldo Rivera did a show from a nudist camp early this month. Even the "USA Today" television magazine reaches for zesty topics under the guise of reportage.

Last week it featured, for instance, a report on nudity in sleaze cable TV shows, including, naturally, film clips of the scenes "USA Today" was deploring.

- A THIN LINE SEPARATES trash TV and legitimate education. The difference is less than the choice of topics than how, why, when and by whom they are addressed. Decker told the caller his guests were recognized experts who were discussing a genuine problem. That was a good answer. The show aired at 10:30 p.m., a reasonable time for the theme.

The two guests were Donald Strassberg, a University of Utah psychology professor who specializes in sex education and therapy, and Freida Stuart, a social worker who runs the sex and marital therapy clinic at the university medical school. They are two of the three certified sex education counselors in the state.

Strassberg had been on the Decker show twice before, discussing sex education and pornography, two subjects on which he has liberal views, and often talks on radio and TV news shows. The discussion of sexual difficulty was his first on TV here, however, and the most candid I have seen on a local program.

Strassberg did a call-in program on K-Talk two-way radio for awhile. Explicit radio sex-advice programs have been fairly common for some years, most prominently "Dr. Ruth," who also has a cable TV broadcast.

- RESPONSIBLE AND ENLIGHTENED discussion of even the most sensitive subjects is a welcome development, though any discussion at all of sex is bound to displease some persons.

Strassberg sees radio and television as effective avenues of effective sex education, to help people understand problems and to put them in reasonable perspective. Orgasmic problems, he says, are the single most common female sexual complaint, affecting some 85 percent of sexually active women. Strassberg says he doesn't attempt to give specific advice on individual problems over the air.

Most responsible media now recognize that we can't deal with personal and social problems when they are swept under the rug and are talked about, if at all, in hushed tones. Despite the dangers of exploitation, and despite the risk of offending some readers and viewers, opening up previously taboo subjects has overall been a healthy trend. It was some years into my professional lifetime before reporters were able to use the words rape or syphilis, for example, much less discuss them intelligently.

Two years ago it would have been unthinkable for the general press to use the term condom. This month the respected magazine "Consumer Reports" has a cover story blaring "Can you rely on condoms?" and seven pages appraising condoms and their uses. It also has a page clearing up some misconceptions about AIDS and advising readers where to get more information. The AIDS threat, of course, made discussion of protections inevitable - and wise.

- PRIVACY AND SEXUALITY in many forms continue to be sensitive problems for the press.

The New York Times last week carried a story obituary about Oliver W. Sipple. He is the man who thwarted the attempt to assassinate President Ford in San Francisco in 1975 by knocking a pistol from the hand of Sara Jane Moore.

Tucked six paragraphs down in the obituary was the second news point, that the Sipple case had ignited a debate among news groups about rights of privacy versus rights of the press.

Sipple said he suffered from becoming an instant celebrity because news accounts identified him, accurately, as a homosexual who frequented gay bars. He sued the newspapers, magazines and news agencies for the embarrassment and humiliation he said he suffered.

Though courts have ruled that the media can infringe privacy by dredging up irrelevant facts about a person, someone involved in a news event even inadvertently cannot win on a complaint of intrusion. But Sipple had a legitimate grievance even if he did not have a legal case.