When German health institute Epicure questioned some 1,000 men between 31 and 69 about their sexuality, it discovered that one in five men fantasizes about having an affair. But the heavier he gets, the less likely he is to fancy a fling.

So despite his obvious advantages, a man with six-pack abs does have his flaws.

The impotence issue

"The more you weigh, the less you'll feel like sexual dalliances," said Sascha Rusch from Epicure.

But do fat men just have fewer opportunities for hanky panky, or is their sex drive really lower? Apparently so. Seven percent of men enjoy sex with prostitutes, in comparison to just four percent of overweight men.

Low self-confidence is partly to blame, but Rusch pointed out that the main reason is more likely to be lack of exercise and bad diet.

"Only 44 percent of men believe in eating healthily," he said. "But there's an immediate connection between obesity and impotence."

Social standing

If even getting up off the couch seems like an effort, bedroom frolics are probably the last thing on a man's mind.

But the study also showed that 75 percent of men are willing to tackle lack of energy with regular sport -- even if they don't necessarily do so for the benefit of their better halves. Four of five men said they lead a healthy lifestyle in order to live longer and feel better, 55 per cent of them say they hope personal fitness will improve their self-confidence, while only 27 percent of the respondents said they worked out in order to appear more attractive.

"Ego tends to be the main motivation," Rusch said.

Even so, the older a man is, the more he earns and the better his education, the more effort he'll put in to pleasing his partner.

Whatever the reasons, we now know that fat men are faithful. But the inevitable question is: Are their partners faithful to them?