They studied 22 men in Buffalo, NY that smoked Marijuana 14 times per week for over 5 years. They were found to have decreased seminal fluid, decrease sperm, a decrease swimming ability. The result is a decrease in fertility. They were tested against 59 fertile men that were able to cause pregancy.
http://www.thebostonchannel.com/health/2551085/detail.html?treets=bos&tml=bos_health&ts=T&tmi=bos_health_1_12150110142003
http://www.thebostonchannel.com/health/2551085/detail.html?treets=bos&tml=bos_health&ts=T&tmi=bos_health_1_12150110142003