As far as male fertility is concerned, length really is important...except the length in question isn't the one you're thinking about. The crucial distance is actually the length between scrotum and anus. Yes, it's all about the taint.
In what has to go down as one of the most heroic scientific experiments ever, University of Rochester researcher Shanna Swan measured the taint length of 126 men born after 1987. The average anus-to-scrotum distance is two inches, and according to this study, you do not want to be below average.
Swan and her team found that men with below average taints were 7.3 times more likely to have low sperm counts than their above average counterparts, and the sperm they did have was not as well-formed as those belong to men with longer anus-to-scrotum distances. And why is all this happening? Well, we do know that men have taints that are roughly twice as long as those of women, suggesting a longer taint correlates with greater virility and male fertility.
Honestly, all I can really think to do here is to remember this slightly NSFW Mr. Show classic, which I can't help but think has much to teach us about the topic at hand.