You know that characters in a TV movie are about to do something naughty when you hear a saxophone on the soundtrack. In "Sharon: Portrait of A Mistress" on NBC Monday night, married man Ed Dowling and employee Sharon Blake found themselves conspicuously alone in Cabin No. 2 at a ritzy resort. Just Ed and Sharon - and a saxophone.

Ed looked at Sharon.Sharon looked at Ed. And right on cue, that old devil saxophone began bleating guilt in the background. Saxophone, thous instrument of evil! In the 50s and '60s they couldn't make a movie about dope addicts or beatniks without the official sanction of the soundtrack sax. Now it's the official accompaniment for thoughts of lust.

But we are not concerned with sax on television. We are concerned with sex on television. It there any? Is there enough? Is America getting an immoral fiber diet of lascivious fantasies?

The answer to all these questions is the same. Yes and no.

There are indeed more programs with sex-related themes this season than last season - from the attempted rape of Edith Bunker to the bedroom farce of "Soap" to NBC's "79 Park Avenue," which might have been called, "Marja: Portrait of a Madame." NBC began its Monday night movie season with a few romp called "Sex and the Married Woman."

Meanwhile regular series are doing their sex numbers, too. Sample story line from ABC's "Eight Is Enough"; "Tom disapproves when oldest daughter Mary moves into an apartment with her "fiance" without being married."

Sample story line from ABC's "Carter Country"; "Chief Roy is dumbfounded when veteran teacher Bill Peterson, one of his best friends, announces he is gay and is fired by the school board."

There was even hokeypokey along "The Oregon Trail" on NBC; "Stella" Stevens guest-stars as Hannah Morgan, the leader of a band of shady women posing as mail-order brides."

All this titillating material available to viewers brings up another problem - how you gonna keep 'em down on Walton's Mountain, after they've seen Cheryl Ladd in a bikini? Well, you arent. "The Waltons" ratings are way off this season, but "Charlie Angels" bounce happily along.

The all-too-easy conclusion to draw, and it's already been drawn to death, is that because networks are down-playing violence this year, they are forced to up-play sex. Equating sex and violence is specious, in the first place. Even if the theory were true - and it practically presupposes a conspiracy on the part of networks to corrupt us - it's hard to see how an abundance of sex talk on TV could be as potentially harmful to the national psyche as the over-abundance of violent acts that only lately has been letting up.

There is absolutely no proof that sexual innuendo on television has ever led to the warping of even the most warpable mind nor caused people to veer off on mad carnal benders. The fact remains that if sex talk and sexual themes have increased on network TV this year, there is still no sex on television.

"All we're doing really is alluding to sexual relations or alluding to sexual activity," says Van Gordon Sauter of CBS, probably the most civilized and frank of the network censors. "You never actually see any sexual activity on television. I've traveled around the country a great deal, to colleges and various cities, and believe me, television is still incredibly conservative when it comes to sex."

Sauter maintains that the amount of sex-related programming on CBS is actually down from last year but says that on the networks as a group, "the wall has moved a bit" toward greater frankness. "But not," he adds, "to the point where it has offended the majority of television viewers. Last Tuesday night, 38 per cent of the people watching television at 9:30 were tuned to 'Soap'. You can say it's evil or unfortunate, but that's apparently what these people want to watch."

Sauter says he disapproves of "Soap" and would still turn it down for broadcast on CBS, 38 share or not.

In fact, although much of the "Soap" clamor by activist groups has died down, "Soap" has been cleaning up its own act lately. Not only does there seem to be less talk about sex on the show, there is also an obvious movement afoot to make the "Soap" characters more human, less harsh and mechanical.

On next week's episode of "Soap," previewed for the press as evidence that there's been reform, Jody finally convinces his Mafioso brother Danney that he's really "gay" and not just "shy" (they stole a joke from Lily Tomlin for that one).

"Face facts, will you, Danny?" says Jody, "I'm a homosexual."

"But - you're too good in sports," Danny protests. At last he accepts the truth, and the studio audience cheers when he hugs little brother forgivingly. Thus has "Soap" turned to mush.

"Sharon," which carried no network disclaimer about "mature audiences," was by no means sexually explicit, but there are levels of implicitness, and it may have broken through to a new and raunchier one for prime time. The dialogue was occasionally smuggly suggestive.

She: "I want to go jogging, need the exercise." He (wickedly): "I'll give you some exercise."

Later, they meet in the front seat of his car while she is still in her tennis togs, and he proceeds to snuggle imploringly. She says, "I'm all sweatly." How television has changed since "The Adventures of Ozzie and Harriet."

And yet despite some corny dialogue and its patently sensationalistic title, "Portrait of a Mistress" was not a bad title TV movie; it even seemed at times to be groping for truths about life among the sexes. What is less defensible than increasing candor is the smutty way it's being peddled. Commercials promoting the show made it look like a sleazy splash of trash.

Print ads for the upcoming CBS telecast of Jacqueline Susann's "Once is Not Enough" promise viewers a "sensational motion picture" that "explores the dark alleys of jet-set love," and lets us all merrily "step into a world where everything goes - beginning with innocence."

"I have nothing to do with the advertising of the product," says Sauter when the CBS ad is read to him. "Sometimes they'll bring me an ad before they run it. I just groan in anguish. Then we try to reach some kind of compromise. Generally, we're very responsible about it."

For all the talk-talk-talk about sex-sex-sex, however, network television programs still invariably give a rousing endorsement to traditional middle-class morality. "Television is not and does not want to be a trendsetter of the sexual revolution," declared NBC vice president William S. Rubens in an October speech, Sauter agrees.

"We have a big group of people out there in that audience who have a lot of different attitudes," he says. "The bottom line is that a television network is not there to carry the banner of sexual change or to promote new forms of sexual conduct. Viewers don't want entertainment programs telling them what proper social conduct is any more than they want news programs telling them what proper political conduct is."

And so what television essentially espouses is the old C. B. DeMille 11th commandment - characters can misbehave their heads off, so long as they are punished for their wicked ways. Sharon, the portrayed mistress, had her kicks for two hours, but it was clear at the fade-out that she had exiled herself to a zombie life of shrill thrills. Never would she know the true happiness of those smiling moms in the cake-mix commercials.

Meanwhile, the flood of sexual innuendo rises in all media, not just television.A mattress firm recently offered readers of a New York newspaper T-shirts that say, "I only sleep with the best." A stereo company is about to begin a multi-media campaign pegged to the line, "Please be kind; this is my first time." Outright pornography - which generally is not thrust upon unwilling customers - gets all the indignant attention, but the innuendo surges on.

Will television ever break the suggestion barrier and cross over from implicit to explicit in dealing with sexual matters? Cable TV yes, and pay TV as well, but though network TV's obsession with sex may go on as long as it sells, putting me is in no way dependent upon shutting up.

Or, as Sauter says, "Thank God I'll be in some other job long before somebody has to cope with that madness."