Different species, same problem
Changes in sperm motility and damage to the DNA in the sperm are issues for males of both the human and the canine species. Over roughly the last 75 years (from 1938 to 2011), researchers have documented a 50-percent reduction in sperm quality in humans and the trend occurs worldwide. Similarly, dogs that live in homes with people have exhibited a sperm quality decline of about 30 percent over a 26-year period. These changes over time suggest that the declines are due to environmental factors.
via Whisker Therapy