Your balls are outside your body for a reason, and the reason is this: It’s good for your sperm. Your swimmers develop best at temperatures two to three degrees cooler than the rest of your anatomy, hence the evolution of the unseemly bag of skin and muscle that you lug around all day with your Treo and iPod. Reptiles don’t have to do this, nor do birds, their reproductive organs being discreetly tucked away, like Murphy beds, inside their bodies. But you’re a mammal, and to be a male mammal is to have balls—and to be a human male mammal in the 21st century is to fret about their ability to function.

Witness one guy who was trying, with his wife, to conceive a child. Things weren’t going too well. Months of zinc tablets and loose underwear passed. No embryo. Then the guy discovered that his office was relocating. He would have to schlep to his new workplace from a parking lot that was a 15-minute walk away. That journey would heat up his testicles to a point where he’d be as likely to fertilize an egg as Mel Gibson would be to host a seder. The guy lobbied for a VIP spot. He got the parking place but, sadly, no baby.

This is exactly the type of story being told over and over again at cocktail parties and watercoolers as we speak.

“I was at a drinks thing the other night, and a former coworker said, ‘Everyone, I’m sorry, I’ve gotta go knock up my wife,’” says Andrew Nathan, 34, a group account director at an advertising agency in New York. “I was slightly surprised by it, but I guess I appreciated how direct he was.”

“You talk about fertility with the guys, there’s no doubt about it,” says Jacob (not his real name), a 35-year-old real-estate executive in New York who recently cut short a night out with buddies to get home to his ovulating wife. But the fact that the subject has become fodder for bar-stool conversation isn’t necessarily a positive development. “The freedom to talk about conception—it puts a couple through a lot,” Jacob says. “You’re trying to conceive. It’s stressful, and your friends are all ‘Have you made a baby yet?’”

If you’re among those attempting to join BabyBjörn Nation, things are about to get even more stressful. Doctors working in the $3 billion–a–year infertility business are waving some unnerving statistics in the faces of men trying to get their wives pregnant.

“Even guys as young as their late twenties and early thirties should be worried about their fertility,” says Harry Fisch, director of the Male Reproductive Center at Columbia University Medical Center and author of The Male Biological Clock. According to current statistics, infertility affects around one in six couples. Contrary to the common assumption, in about a third of these cases, the problem lies with the man. Studies over the past 20 years suggest that the quality of semen has declined throughout the industrialized world. The American Society for Reproductive Medicine recommends that sperm banks have a cutoff age of 40 for donors to “minimize the potential hazards of aging.” And the real kicker? Men over 35 are twice as likely to be infertile as those under 25.