Ignoring the occasional guffaws and snide remarks, men are pulling on their court shoes -- and pulling in their paunches -- to hie off to what was once considered women's turf: aerobics classes.

Though men are still outnumbered by legions of leotards, they're becoming a regular sight, sometimes making up about one-half of the class enrollment. Some are even teaching.

"I feel sorry for guys that aren't man enough to exercise with women," declares National Cancer Institute senior scientist Cecil Fox, who has participated in aerobics and other fitness classes for 15 years.

"Americans have the mistaken idea that big muscles mean you're in shape and that football players are in good shape. You can take one of these weight lifters and put him in an aerobics class and he simply can't keep up," adds Fox, 50, who has also taken classes in Europe, where work in cancer research has taken him. "Cross-country skiers and orienteers are in better shape.

"In Europe, the attitude is not that this is some kind of funny stuff that girls in strange uniforms do. It's perfectly all right for men and many men take classes."

Other support for men's efforts comes from fitness experts, exercise surgeons, artistic directors of dance companies. Even the U.S. Army officially recognizes aerobics as a valid means of exercise for its soldiers.

Elsewhere, 45- to 60-minute workouts suited specifically to men's needs and usually accompanied by music include exercises that place greater emphasis on tightening upper abdominal muscles and increasing flexibility of the hips by stretching muscles left tight by jogging, squash, tennis and racquetball.

"Aerobics is changing," says environmentalist David Burack, 44, who began teaching a men's aerobics class this month at Capitol Hill Squash & Nautilus Club.

"A lot of people like to say it's defined by Sorensen and Fonda, but it's developing very fast. We're seeing specialization -- like in my class."

To improve "flexibility, agility, balance and speed," Burack's routine -- geared to men age 30 and above and also open to women -- calls for exercises "reducing the midriff and emphasizing shoulders and arms."

"A lot of men are preoccupied with the macho image," said a sweaty Sam McFarland, 42, after he helped teach a recent aerobics class at Walter Reed Army Medical Center's recreation hall. "They think 'twinky' or sissy or something like that."

McFarland, a 6-feet-3, 217-pound administrative officer, is one of about a dozen men and women who team-teach classes offered daily by Heavenly Bodies, a company that has taught Walter Reed employes for two years. Founded by Corinne Delaney, Kensington, the firm designs corporate-fitness programs for Washington-area organizations.

"The hardest thing," says Delaney, "was stressing this was a masculine class," when she started the company in 1982. Today, men make up from 30 to 50 percent of the classes her 14 instructors lead.

It makes for a significant increase from the days when Cecil Fox was their "first man," says Debra Gault, 28, Delaney's full-time instructor and vice president. "It took a long time to get the men to come over."

"When a guy comes in," says McFarland, who has taken Delaney's class for two years and instructed for more than a year, "he thinks it's a piece of cake. But as the class goes on, his respect increases.

"Initially," concedes McFarland, "you wonder if you're going to be labeled a sissy if you go to this class, but there are no cutsey-cutsey steps. Women challenge the men. They don't want wimps in the class."

Rather than take offense and pretend that aerobics is female territory, women tend to be supportive and encourage men to participate.

"The thing I find so funny is that this is something a lot of men really need," says Kathy Rowe, a YWCA fitness specialist. "They are stiff in the hamstrings and the lower back and they have no exercises to stretch out. Most of them didn't do much in flexibility training unless they've done running and track.

"The other surprise is that they think it's going to be all dancey and prissy, and then most of the time they're overwhelmed what kind of a workout they get."

Men's reasons for taking the class are often similar to women's. McFarland wanted to lose weight resulting from his desk job, as did Fox, who blames his weight increase on English pub lunches.

"The class is a good compromise between becoming an athlete and turning into a great big blob of jelly," says Fox, who claims he is "not athletic," but enjoys the diversity of aerobics activities.

"It doesn't do weird things to the feet and knees, it exercises the entire body. I think it's strange to see a bunch of guys running down the street together. Some of the military may have problems with it -- maybe it's not macho enough for them."

The U.S. Army, however, several years ago designed its own aerobics and total fitness program. Although the classes at Walter Reed, taken by its military and civilian employes, are not exactly the same routines as those the Army created, Delaney and her instructors draw on military exercises and have put troops in good enough shape to pass the Army's twice-yearly physical readiness tests ( necessary for promotions).

"This particular class is a much more carefully thought-out approach than most," says Col. Harlan Baker, 48, in charge of troops on a remedial physical training program. "I also love to come in here and see people warming up."

The Army's program, which places special emphasis on those age 40 and above, allows each person to progress at a self-appointed pace. The routines and advice on nutrition and health habits for the general public are detailed in The U.S. Army Total Fitness Program: Be All You Can Be! by Dianne Hales and Lt. Col. Robert E. Hales, M.D. (Crown, $14.95).

But sometimes the incentive to join a women's aerobics class is prompted by more than the pursuit of physical fitness.

Says one 36-year-old single economist who takes an aerobics class at Washington Squash and Nautilus Club:

"I'd already been giving it some consideration. I was looking at the aerobics instructor. She had a great body in general, but a great body in shape is even better.

"She was looking quite good -- that was the final straw. I signed up."