WHY DOES A married woman take a lover? W How does she go about it? What does it do to her marriage?

These are questions that Elaine Denholtz, a member of the English faculty at Fairleigh Dickinson University, set out to answer in interviews with more than 100 mainly middle-class women who have had lovers. Her book, "Having It Both Ways," (Stein and Day), is a fascinating exploration of a phenomenon we are bound to be hearing more and more about as women surge into the workforce and find that they have more and more opportunities to take lovers and more and more reasons to do so.

The women Denholtz has talked to are not passive. They are assertive women who found that their marriages did not fulfill their needs. Often their husbands were uncommunicative and insensitive lovers. Some women, borrowing a page from the unfaithful male's book of excuses, said they were bored. One did it to get even with an unfaithful husband. Some said their affairs harmed their marriages while others said their affairs made their marriages bearable. Some affairs lasted a few months or a few years. One lasted 16 years before her husband learned of it. The most striking conclusion, according to Denholtz, is that the women felt they were entitled to their affairs.

"Feminism has raised expectations," says Denholtz, "and has taught women they are valuable and they deserve happiness. They're not going to sit around for 30 or 40 years in a lousy marriage."

Statistics on marital infidelity differ. A recent book suggested that one out of every two men is unfaithful. The Kinsey report of the early 1950s suggested that one out of every four wives had an affair by age 40. The 1978 Redbook Report on Female Sexuality found that one of every three married women has had an affair. Denholtz believes that one out of three is close to the truth. The fact that she had little trouble finding women to interview suggests that extramarital affairs may be much more prevalent than generally assumed.

She interviewed women of all ages, from the 20s to past 60, and they are identified by fictional first names. They were women she found by word of mouth, through other women. The women tended to care deeply about their husbands and children and went to great lengths to keep them from discovering their affairs. "Most were very respectable women, women who held down jobs, and were good wives and good mothers," says Denholtz, "but they had needs not being met in their marriages.

"Almost every woman felt guilty. Women have been socialized to believe it's okay for men, men are natural philanderers. Two women said they were told by their mothers to expect their husbands to fool around. Women, as their affairs take hold, feel less and less guilty. One said it was an oasis. One said the affair took the heat off the marriage. She was never so pleasant and nice. Another said she felt smart, sexy, released. Their lovers saw something in them as a person that their husbands may not have seen."

The women talk candidly about their husbands. A married woman reading this book will recognize moments in her own life, whether she has had an affair or just thought about it. Men reading the book will discover that things they think are funny strike their wives as boorish. They will gain insights into what women want. And they will find out that women, like men, are saying their spouses don't understand them, and finding understanding elsewhere.

Take the case of the wife who worked as a court stenographer. She had surgery, came home from the hospital uncomfortable from the surgery, with her hand in a cast, to find a week's dirty dishes, the house a mess and the laundry waiting for her. Her husband had fed the children, nothing more.

"I think he expected me to come home from the hospital and get right back to work. He wouldn't allow me time to recover. I felt so let down, so disappointed. I could taste the anger in the back of my throat . . . . I wanted to hit him.

". . . It was Arthur's total lack of empathy that drove me into another man's arms. I was hurt. I felt short-changed. I wanted a man who would be there to comfort me."

Denholtz found that for most of the women sex was not the enduring attraction of their affairs. "It's the emotional bonding. Women find it very hard to have dispassionate sex. Over and over, they said it's very hard not to get emotionally involved.

"If a man finds out his wife has someone else, his first question is, 'Did you sleep with him?' That's the bottom line. With a women, her first question is, 'Do you love her?' "

Denholtz's book is mercifully unjudgmental. What she has done is find the women behind the statistics and give them voices, instead of pretending, as does society at large, that there is no such thing as an unfaithful wife, or if a woman is unfaithful, it's because she has a rotten marriage or she's mentally ill. By taking the phenomenon seriously, by giving these women a chance to talk about what they have done and why, she is bringing forth a subject that has been kept in the closet too long.