Obesity is like a natural contraceptive. Not only does it make you less attractive to the opposite sex, but men with bulging bellies also make less sperm.

And in plenty of cases, they make none at all.

It's been proven before, and now it's proven again as French researchers say their analysis of 14 studies shows that as waistlines grow, sperm counts shrink.

Those 14 studies involved a combined 10,000 men -- men of different sizes and shapes, all of whom provided ejaculatory samples for comparison. (Side note: Pity the life of a poor research assistant.)

They found that about a quarter of all men have low sperm counts, and that obese men are 42 percent more likely to be among them than normal-weight men.

Even more frightening, the obese are 81 percent more likely to make no sperm at all.

Overweight men, guys who haven't reached the obese zone yet, were 11 percent and 39 percent more likely than the normal weight guys to have low or no sperm -- but let's face reality here: If you're badly overweight, you're probably in no condition to be "doing it" anyway.

All that huffing and puffing makes sex more work than pleasure, and possibly dangerous work at that. And that's even assuming you can get it up in the first place, because obesity kills erections.

That's because extra body fat has the unique ability to turn testosterone into estrogen, and once that excess estrogen starts flowing through a man's body he may as well start knitting -- because he's not nearly as much of a man as he used to be.

Want to avoid all that? Then lose the pounds now.