Macho myths notwithstanding, no man can long do well by two wives, especially if his first "wife" is his business.

Indeed, Boston psychiatrist Barrie S. Greiff finds sexual problems "very common among businessmen and professionals -- anyone who is deeply committed and intensely involved in his work." Sexual problems also occur in women with business responsibilities, especially when those responsibilities, and the rewards for assuming them, exceed those of their spouses.

Greiff, who has been psychiatrist to the Harvard Business School for 14 years, says the most common expression of a man's sexual difficulties is lack of interest in his bed partner. Another is involvlement with someone else, sometimes with a woman in the office. Many men complain of inability to relax enough to be able to perform sexually. Specific problems include impotence, premature ejaculation, and inability to reach orgasm.

Perhaps typical of how these probalems emerge is the case of a 47-year-old man who, Greiff says, was "really up to his ears" in the business he had started. Though he was "getting clobbered from every angle" by business and economic pressures, he was handling the situation.

What he couldn't handle was his feeling that he was "unappreciated, misunderstood, and imposed upon by his wife," Greiff recounts. "Without being aware of it, he was enraged at his wife and was retaliating by using sex as a weapon. In effect, he was saying that if he couldn't share his nearly over-whelming problems with his wife, then he sure wasn't going to share his body with her. The result was noncommunication, coldness, inattention, and lack of sexual interest. This is turn made her upset and angry.

"When the husband came to see me, he never actually told me of his anger but complained instead that 'Things just aren't as much fun anymore.' During a visit when his wife was present, he finally described what he was feeling. His wife was dumbfounded: 'How was I supposed to know that?' she protested. 'You were trying to protect me by not telling me anything, but then you were angry at me because I didn't understand what you were going through!"

That exchange, says Greiff, "really opened up the floodgates and relieved the man's crushing sense of 'having no one to share it with.' That was the breakthrough. After that things began to get better."

Greiff has identified several early signals that a couple's relationship, both in and out of bed, is headed for trouble or is already in trouble. They include reduced frequency of sex, lack of pleasure in it, boredom, and concern over performance, especially if that has never been a problem before. On the other hand, frequency sometimes increases in a troubled relationship, because the couple is trying to prove to themselves that everything's still OK.

When Greiff finds that a patient is having a sexual problem, one of the first things he does is find out if there is a physical basis for it. Output of sex hormones, for example, can be reduced by disease. Or a mental disturbance is sometimes involved; depression is the most powerful damper on sexual activity.

Much less of a factor, if it's a factor at all, is the so-called male menopause. Though there may be some falling off of hormone production and sexual responsiveness when a man goes well past the middle years, medical specialists agree the changes won't prevent a very adequate sex life.

It's important for a man to recognize, Greiff says, that "sex is subject to change. It's not the same as when you were 20, it's not the same when you hit 40, and it's probably not going to be the same when you're 60. It may actually be better as you grow older!In sex, as in anything else you do, you can get burnt out -- unless you build in a kind of creativity."

By creativity in sexual life Greiff doesn't mean just trying new positions. Rather, he means making changes to "perk up" the whole relationship, of which sex is only one part. "That doesn't necessarily mean you have to rebuild your relationship, but you do have to retune it."

Steps that can help improve a relationship and engender closeness and warmth can be very simple and unexotic. They might be as simple as going away together for a weekend, even taking a long walk or going to a movie. Something as whimsical as checking into a motel on an impulse might help to rekindle old flames. The point is to make time and space to do things together that you don't ordinarily take time to do.

Greiff traces many of the difficulties men experience during sex to attempting it at the worst of times -- for instance, when they're preoccupied with other concerns or just plain exhausted. While it's impossible to define the best time for sex, "a good time is when you're relaxed enough to enjoy it, when you don't have to rush, when it's not squeezed in between Johnny Carson and going to sleep."

Sexual problems, he adds, "are almost always reversible. Moreover, many of them disappear on their own. Just talking about them with your spouse may be very helpful. But if this doesn't work, I think it's best to talk to a physician, because of the possibility of physical or mental factors being involved."

In the 1980 book, Tradeoffs: Executive, Family and Organizational Life, Greiff and co-author Preston K. Munter, psychiatrist to Harvard's Law School, underscore the erroneous assumption of many businessmen "that their marriage will prosper no matter what happens, merely because their spouses don't complain unduly." Their conclusion: "Marriage is no different from business: They both require constant attention and careful appraisal."