Adultery, a sin to some, a fact of like to others, is unquestionably a sensititve subject. Most people wil say they feel infidelity is wrong, but if we look closely at the statistics on extra-marital affairs it's clear society isn't practicing what it preaches.

One reason for this social dichotomy is that adultery in itself is full of contradictions: the "I-know-it's-wrong-but-I-feel-so-wonderful" syndrome, with guilt and pleasure in constant struggle. Then there is the secrecy and masquerade that traditionally accompany adulterous affairs.

But something new has developed. According to recent studies, more married women are having extramarital affairs, with as many as 40 to 50 percent of wives said to have an affair by age 40. This is dramatically close to the classic Kinsey figure of 50 percent for married men.

Ironically, this knowledge could bring adultery more into the open. Social scientists want to know why this is happening and women are responding to their questions.

According to a report by Dr. Frederick Humphrey, former president of the American Association for Marriage and Family Counselors, the "typical profile of a woman in an extramarital affair": She is in her mid-30s, middle class, with at least one child, married approximately 13 years.

In 1974 Dr. Lynn Atwater placed this ad in Ms. Magazine: "Married women involved now/recently in extramarital relationships: feminist sociologist desires personal or mail interview for thesis. Strict confidentiality . . ."

From 300 respones to the ad, Atwater, now associate professor and chairperson of the Department of Sociology and Anthropology at Seton Hall University (South Orange, N.J.), selected 50 women to participate in a study which resulted in an upcoming book, The Extramarital Connection: Sex, Intimacy and Indentity (Irvington Publ., $15.95).

"My premise was to look at women who have changed their lives . . . or who wanted to change their lives," says Atwater, 45, who sees the rise in women's extramarital affairs as part of their new role in the job market. Bluntly put: Mother is out working, meeting more men, finding out who she is, and risking divorce because she can support herself is she has to leave her husband.

"These were not rabid feminists," says Atwater. "I tried to get variation in age (23-59), with zero to numbers of children, those unhappily and happily married, with marriages that are split and together, those involved in long and short relationships."

Because most were actively involved in other aspects of their lives -- career, school, family -- they met with their lovers only once a week, or less. The affairs were essentially considered just another part of their changing life styles.

Unlike most other researchers, Atwater included questions about the positive side. "If you're going to understand a phenomenon completely you have to look at both sides. I was trying to be morally neutral, not to condemn it from the beginning . . . I didn't know when I started working with the women that I was going to get such ppositive reponses."

The No. 1 testimony repeated over and over by the 50 women was their need for intimacy, which they said they found with a lover.

It was a very long-term closeness with another person. I wanted -- and it seemed very natural to -- to extend that closeness to include sex.

It meant having another friend.

I guess essentially what I get is the companionship, the closeness and the ability ot communicate with a male.

Their husbands, these women said, were "inexpressive." The most important thing their lovers did was "communicate," or as one lonely housewife put it, "He made me feel that I was special."

As the book points out, it's not just an easy problem of husbands not talking to wives -- other studies have shown married men involved in affairs also are looking for companionship and "someone to talk to."

Though many of the women commented on how difficult it was to find the right words to express their feelings, they "specifically rejected traditional romantic feelings, as well as the traditional words 'in love.'"

They were not at a loss for words, however, when it came to comparing their sex life at home and with their lover.

"Women found their extramarital sexuality to be different because their lovers gave them specific sexual experiences they did not or could not get at home," write Atwater. "Interestingly, a few women felt a sexual constraint with their husbands that they did not feel with their lovers."

"There are no solutions in the book," says Atwater. "Obviously it's going to be a long-term problem, but I don't see how social trends are going to decrease extramarital behavior. In fact, every scholar in the area feels there's going to be an increase because the people who were premaritally permissive in the '60s and the '70s are now married and all these studies show that the more premarital behavior the more extramarital . . .

"All this besides the factors of mobility, freedom and the need for intimacy. This is just the beginning of the discussion."

Melissa Sands is as full of surprises as her recent book, The Making of the American Mistress (Berkely Books, $2.50).

Yes, she was a mistress of a married man -- the term "mistress" does not mean "kept," but intensely involved" -- and in 1979 wrote The Mistress Survival Manual . She is the founder of Mistresses Anonymous.

But after all that, Sands is not in favor of adultery and is building a career of helping women who are unhappy about their love triangles.

"I hear from thousands of women," says Sands, who is in her early 30s and married for two years to her former lover. "I know it was no picnic; I won't do it again."

The women in Sands' book are a desperate lot, seemingly crying in unison, "How did I ever get in this mess?" and "How can I get out?"

"It's because of passion," claims Sands. "We have a need and an appetite for passion and you get a lot of hype in society saying the only way to fulfill your quota is to have an affair. What they don't tell you is that yes, your passions are going to be filled in terms of romance and intrigue, but your passions are also going to be filled with frustration and rage."

Sands' book covers the emotional highs and lows of the single, married divorced, widowed, young and old mistress, but the chapter on the married mistress plays up the adulterous wife's "double-trouble" situation.

"Just in the negotiating all the variables of where you are and where you really should be, where your children and your husband will be when you're meeting your lover . . . it's time-consuming and you're not really analyzing where it's going to go when your husband finds out," warns Sands.

"We don't make the practical calculations, we only make the romantic calculations."

As for the woman whose husband prefers to read the paper at night rather than talk, and who falls asleep on the couch . . . "Well," says Sands, "when you get in an affair with a married man he's going to be more charming than your husband . . . he's going to be more attentive, more affectionate . . . but that doesn't mean that you should alter your life for him, because if he does become your husband he's going to be just like the first in those ways."

Sands' women, unlike those in Atwater's study, are torn between leaving their husbands and waiting for their lovers to leave their families: in mistress mythology, "an impossible dream."

Sands believes strongly that although women start out feeling they are not going to get intensely involved, they do.

"You only see each other at your very best . . . totally alone, the clandestine nature makes it more exciting . . . it's psychological self-disclosure . . . he tells you things, you tell him things and because you're alone and have no one but each other . . . you become closer."

Though Sands is definitely a crusader for fidelity, there is a depressing tone when she speaks of today's marriages.

"Women are very romantic. We're born and raised on romance. The only problem is we don't teach our daughters how to keep romance going in our marriage. We don't plan an hour with our husband once a day or once a week, like lovers have to . . . if you want a monogamous life style you have to make it a priority. You can't just look the other way.

And "Oh yes," she adds, "it's just as much a man's job."