Because a man can perform sexually is not a guarantee of fertility.  

From the guardian.ca.uk "Men should not smoke, drink or take unnecessary drugs if they are planning to become fathers to avoid causing health problems for their children, a health expert has warned.  Scientists found that toxic chemicals can damage sperm, which then pass altered genes onto babies. In experiments on rats Matthew Anway of the University of Idaho found that some garden chemicals caused problems such as damaged and overgrown prostates, infertility and kidney problems, all of which were present up to four generations later."

Oftentimes, there are no signs of infertility.

"the problem may lie with the sperm itself. Anything affecting its quantity (a normal sperm count is more than 20 million sperm per milliliter of semen), quality (curled, crooked or two-tailed sperm are less effective) or motility (about 60 percent of a normal man's sperm show good forward movement) can affect a couple's chances of conception. "

Several factors contribute to male infertility such as varicoceles (enlarged scrotum veins), excessive heat, the man's age, presence of lead and pesticides, and mobile phone use.

Certainly excess body fat contributes to unfavorable hormone balance and influences fertility, too.