When Cathy Greenblat told friends she was writing a book on marriage, their response was almost always: "Oh, is anybody getting married any more?"

"They'd say it with genuine seriousness and surprise," says Greenblat, a professor of sociology at Rutgers University. "It appears that there is a rather widespread belief that the number of people getting married is dwindling."

But Greenblat's research revealed an entirely different picture. "The United States population has been, and continues to be, a marrying population," she says. "Over 90 percent of Americans marry at some time in their lives.

"People are deferring marriage until somewhat later ages -- particularly in large cities -- as more women get the opportunity for education and careers. But the rate for first marriages in 1977 was almost identical to that of 1925 -- and it's still going up."

Although "society continues to place enormous pressure on people to marry," she says, "they do very little to prepare them for the stressful period from engagement to the wedding."

Greenblat became an expert on the ambivalences, guilt and pressures -- both internal and external -- engaged couples face, after writing a textbook on the subject and becoming a "quasi-counselor" to engaged students in her class on "Marriage and Family."

"Popular literature rarely covered the range of emotions surrounding getting married," says Greenblat, who set out to do just that. With psychologist/sociologist Thomas J. Cottle she interviewed about 150 engaged people across the country.

The result is "Getting Married" (McGraw-Hill, $11.95), which examines the romantic, traumatic, myth-shrouded and emotion-charged (negatively and positively) event.

"The idea, and sometimes urgency, of getting married is something that comes from within," she says. "Most people have strong personal beliefs that marriage is the good, right, normal, logical, natural thing to do in our coupled society."

What motivates a couple to marry, however, often isn't so much love as something outside the relationship. "One person might have been offered a new job, want to move, or be fed up with singles' scene.

"Or it could be that all one's friends are getting married, they want to buy a house, or she is pregnant. (Twenty percent of brides are, she says.) sometimes people want to structure their lives, and feel if that one major decision is made, everything else will fall in line."

In searching for a mate and deciding to marry, Greenblat says men are often more romantic and women more practical. "Historically, women have had to be practical. For the vast number of women, the man they married defined the rest of their lives.

"So they thought about marriage at young ages, often rehearsing being a bride. Have you ever seen a little boy dressing up like a groom?"

People either become obsessed over their decision, she says, or "just take take a deep breath and go ahead with it." But once the engagement is announced, a new stress replaces the old.

"At some point during the engagement period, it's common for both people to feel ambivalent. But people feel guilty about wondering 'Am I doing the right thing?'.

"People often think stress only comes from negative events in life. But on a scale of 43 stressful events, getting married ranks seventh -- just below personal injury and just above getting fired."

A major reason for stress and ambivalent feelings "is that many people wait until after they're engaged to start discussing some of the big questions like finances and children.

"They may discover that the other person has totally opposite views in some important area, which shakes them up. Some assume they can change the person after they're married -- which seldom works.

"And it's very hard for people to break an engagement. The high divorce statistics may indicate that some couples haven't resolved these differences by the time they say 'I do.'"

Greenblat, 40, is painfully aware of the difficulty of breaking an engagement. At age 21 she broke one off, a week before her marriage. "I wanted to work, he wanted me to stay home."

When she did marry, three years later, "We decided on Thursday morning and got married on Saturday." That marriage ended, 14 years later, in "an extremely amicable divorce," says Greenblat, who is now living with someone she "might" marry.

"We talk about it," she admits, "but frankly, right now we don't see the benefits that will accrue from it. And it would cost us a lot more in taxes." b

But she denies being "anti-marriage." "I've been asked how I, being unmarried, can write a book about getting married. People write books about death, and obviously, aren't dead.I'm a social scientist, and I've studied -- and experienced -- the subject."

Should she remarry, Greenblat says she'd pay much more attention to "making the wedding service more reflective of who we are. In one of the nicest weddings we attended, the service told about the people getting married and why this union was important to them -- accompanied by music they enjoyed."

In a remarriage, she recommends "doing what is really important to you. In many cases children are involved, so you're not creating only a marriage, but a new family.

"How that child feels about being a part of the wedding should take precedence over a relative who thinks it's shocking to have a child in the ceremony."

The biggest question she says couples face in deciding to marry, during engagement, in marriage and remarriage is "'How much am I still going to be me, and how much will I be someone else?'

"And that," she says, "is unanswerable. People have to find out for themselves."