Exercise improves male fertility, TV reduces it.
Watching too much television can seriously impact your family tree. Scientists have now found that men who watch over 20 hours of TV a week have almost half the sperm count of men who watch very little TV.
Conversely, men who do 15 or more hours of moderate to vigorous exercise every week have sperm counts that are 73% higher than those who exercise little, the findings published in the British Journal of Sports Medicine say. [emphasis mine]
Sounds like natures way of weeding-out us lazy people from the gene pool. :)