We have all had the experience of being told that we need to eat a good, balanced diet. Turns out that's true, at least in order to produce normal sperm.
A study in the March 2010 issue of Fertility & Sterility examined the diets of 30 men with poor semen quality and 31 normospermic control subjects of couples attending a fertility clinic. They recorded dietary habits and nutrient consumption using a food frequency questionnaire. They found that control subjects had a significantly higher intake of carbohydrates, fiber, folate, vitamin C, and lycopene and lower intakes of proteins and total fat. Based on that they concluded that a low intake of antioxidant nutrients was associated with a poor semen quality.
RPMG's Dr. David Meldrum has a lot of information on the role of diet in male reproductive health on his website, Life Choices and Fertility.