Among the universe's cruel ironies—pretty much anything having to do with puberty, say, or the career arc of Michael Bay—this has to be near the top of the heap: Women typically hit their sexual stride in their mid-thirties, just as many guys the same age are pulling up lame with the equivalent of a sex-drive charley horse.

Okay, the sprinting metaphor is a bit much, but you get the idea. Guys experience a huge surge of testosterone during adolescence, but by their late twenties, that jolt is already on the wane. A few years later, women are finally ready to join the party.

Does it get any crueler?

And this situation may be more acute than ever.

"There are a lot of very attractive women in their thirties not getting the sex they want," says Pepper Schwartz, a professor of sociology at the University of Washington in Seattle and author of Prime: Adventures and Advice on Sex, Love, and the Sensual Years. "And it happens as the relationship matures."

Schwartz notes that it can be an off-putting experience for some men—conditioned for years to initiate sex—when their partners are suddenly making the first move. And the second. And the twelfth.

"Men are used to cajoling and seducing, they are used to doing things on their own erotic schedules," says Schwartz. "This often teaches him about his own sexuality—that he's not as omnivorous as he thought." Then something truly bizarre occurs. "Men can get hurt, too, and find women pushy."

Such is the case with "Tina" and "Owen" (some names in this story have been changed), a mid-thirties couple in Brooklyn, New York, who both work in marketing. When they started dating, seven years ago, Owen, who lived in Manhattan at the time, was always up for a late-night outer-borough schlep if it meant he'd score.

"I'd be, 'It's eleven o'clock at night,' and he'd say, 'I'll be there,'" Tina says wistfully. Now married for three years, Owen and Tina are approaching a different anniversary—one year of couples therapy. Let's just say that it isn't Tina's sex drive that needs the tune-up.

"He calls me a 17-year-old boy," she says with a sigh. "It's kind of like a sad joke."

Owen wasn't laughing the day the enormous cherry-red vibrator that Tina ordered arrived in the mail. "I showed it to him—'Look, honey!'—I was all excited," Tina says. His reaction: "That's really aggressive." The vibrator was no fun; it became an issue in therapy.

Tina and Owen are far from alone. There is ample science related to men and sudden dips in testosterone levels—and none of it is good news. Beginning at age 30, most men see the hormone's production fall off to the tune of 10 percent every decade.

And a study last year by the New England Research Institutes found that today's men are manufacturing about 20 percent less testosterone than they were only 15 years ago as they age. Speculation on the cause includes an increase in obesity and a decrease in smokers (smoking, it seems, bumps up testosterone production), but nothing has been identified as the main culprit yet. What we do know are the nasty side effects of these hormonal hard times: more fat, less muscle, depression, exhaustion, and, that's right, a lower sex drive.