Environmental estrogens (Xenoestrogens) appear to play a role in the reduction of male fertility. These estronens are man-made and primarily enter our food supply though the consumption of fish. Man-made chemicals such as PCBs and pesticides have the ability to disrupt endocrine hormones, which affets male fertility. Dr. Michael Greger explains in this short video clip that men who eat fish (which is highly contaminated with mercury, PCBs, dioxins, etc.) have much lower sperm counts than those who eat a plant-based diet.
The process of biomagnification in fish intensifies these contaminates as small fish eat smaller fish and are then eaten by slightly larger fish who get eaten by yet larger fish, and on and on. The toxicity increases greatly by the time larger fish end up on the dinner plate. This is just one more reason why fish is not a health food. Of course other animal products can also effect male fertility.
For more information on infertility, see:
(1) Plant-Based Diet Improves Fertility
(2) How a Plant-Based Diet Improves Fertility
(4) Why Did You Quit Eating Meat?