Society perpetuates two major myths about single people, says writer Jacqueline Simenauer, "each of which contradicts the other and neither of which is true.

"One is the myth of swinging singles whose lives are filled with sex orgies, wild parties and exotic adventures. The other is that singles are lonely, sad people who sit all alone in their small apartments every night, watching TV and sighing."

The reality, says Simenauer who (with co-author David Carroll) spent three years examining single life in America, is that singledom is "a very mixed bag of ups and downs. The greatest advantage is freedom, and the greatest problem is loneliness."

"If singles swing at all," adds Carroll, "it's like a yo-yo, back and forth between exhilaration and the blahs. The predominant attitude toward singleness is ambivalence."

Simenauer and Carroll base their statements on what they claim is "the first nationwide representative survey of singles ever done in the United States." The survey of 3,000 singles aged 20 to 55 in 36 states--plus analyses of the findings by about 50 mental-health professionals--forms the basis of their new book, Singles: The New Americans (Simon and Schuster, 399 pp., $16.50).

The project started in 1978 when Simenauer, who had co-authored two survey-based books about American life styles (Beyond the Male Myth and Husbands and Wives), decided to try the best-selling formula with singles.

"I wanted a co-author who was male and single," admits Simenauer, 44, who has been married for five years and was married previously for three. She linked up with writer Carroll, 40, recently divorced after a 10-year marriage. Before the book was finished, Carroll had remarried. (He insists that the event "had nothing to do with what we discovered about single life."

Approximately one out of every three adults in this country, they note, are single.

"This singles revolution," says Carroll, "is largely the result of social changes we've undergone--the high divorce rate, feminism, the decline in the power of the church, the sexual revolution, the trend to marrying later."

Although most singles consider their life a temporary condition before or between marriages, "We're also seeing," he says, "the truly new phenomenon of those who prefer the single life style. There is a committed group of people within society, about 25 to 30 percent of singles, who chose not to marry."

The survey's most surprising finding to Simenauer was "that single people tend to be conservative. For example, only 20 percent of single men and 6 percent of single women say they're interested in casual sex."

But this conservativeness is often offset, she says, "by an openminded willingness to experiment." The result is often "a strange juxtaposition of new values and old beliefs. You'll hear singles say they believe in living together without marriage but demand fidelity from their partner. Or a man will say it's okay for women to ask men out--but not for the first date."

This occasionally awkward synthesis of old and new philosophies is particularly apparent among single men grappling with the changing roles of women. The survey revealed "some striking differences between what men say and what they do," says Simenauer. "Seventy-five percent of the men said they're indifferent or opposed to having sex on the first date, but two-thirds sleep with a woman on the first to third date and 80 percent sleep with a woman on the first to fifth date."

The reason for this "seeming schizophrenia," she says, is "the strong pressure men feel to perform. Men are very aware that they're dealing with a new breed of women who, they think, are expecting sex. I think that's what's responsible for the new wave of impotency psychiatrists are discovering."

Women, however, express feelings of loneliness more so than men, she claims. "They're more societally conditioned to feel something is wrong with them if they don't have a man."

The single who has the roughest time? "The single mother," says Carroll. "Usually her income has decreased, she feels conflict about work that keeps her absent from her children and she has more fear for her physical safety."

Among Simenauer's and Carroll's other findings about single life:

Meeting

* Almost 40 percent make their initial romantic contact at bars or single functions.

* 40 percent of single women have suffered some kind of physical or mental abuse at a singles bar.

* Most women said they would let a man pick them up, "if the time, the place and the man all meet their particular standards."

* The two biggest obstacles to men are a woman's seeming "unapproachability" and his own "lack of nerve."

* 52 percent of women and 39 percent of men rate intelligence and perceptiveness as "very important" attractions.

* 28 percent of men and 30 percent of women feel a prospective partner's sexual skill is "very important."

Dating

* Almost two-thirds of men and three-quarters of women think "it is all right for a woman to ask a man out on a date."

* Women who have been single for more than 10 years are the least likely to ask men out. Divorced women are almost twice as likely to say it's "wrong" to ask a man out.

* Most women would like to be more aggressive in their dating methods but are held back by a sense of tradition.

* More than three-quarters of men feel women should pay or help pay for a date--but most add the qualifier "occasionally," or "if offered in a sharing spirit."

* Four out of five women under 25 do not want a man to handle the paying, but 30 percent of those over 35 and 40 percent over 45 do.

* 91 percent of men and 93 percent of women say singles of the opposite sex "can and should be platonic friends."

* Half of single men and women stop dating because they find their partners either "dull, superficial, immature or neurotic."

Mating

* Almost two-thirds of men and half of women have sex on the first to third date.

* Over three-quarters of men feel it's "acceptable" for a woman to initiate sex. 31 percent of men say it's a "turn on;" 47 percent say "it's nice" if done "subtly in a feminine way" but a "turn-off" if done in a "pushy way."

* About 75 percent of single men and almost 90 percent of single women feel "it is difficult or impossible to be meaningfully and sexually involved with more than one person" at once.

* Just 15 percent of men and 21 percent of women think love is "absolutely a prerequisite" for good sex.

* Only about one-third of singles rate their lovers as "skilled, imaginative and caring." 38 percent of men and 41 percent of women rank themselves as very good lovers.

Living Together

* About half of all single men and one-third of single women have at one time lived with a member of the opposite sex.

* About 30 percent of men claim that sex is better in marriage because it has time to "grow and mature"; about 29 percent believe that sex is "different" in both circumstances but equally "fulfilling."

* Almost half of single men and a quarter of single women said they would live with a partner without loving him/her.

* 9 out of 10 feel that living together must be sexually exclusive.

* About 75 percent of men and 80 percent of women said they would end a living-together relationship "on the grounds of infidelity."

The Single Life

* Only one-third of single men and one-fifth of single women are single specifically because they prefer the single life style. The rest are single "for personal reasons or because they have not yet found a suitable mate."

* Almost 90 percent of men and 80 percent of women say the problems of being single--such as loneliness, financial insecurity and physical danger--"weigh more heavily on women" than on men.

* Men who live with a group are the first to tire of single life. Those living with a woman are the least likely to tire of it.

* The woman who most enjoys being single is a never-married college graduate, between 20 and 24 years old, in a high-income bracket.