"Frequent and/or long-term marijuana use may significantly increase a man's risk of developing the most aggressive type of testicular cancer, ... Chronic marijuana exposure has multiple adverse effects on the endocrine and reproductive systems, primarily decreased sperm quality. Other possible effects include decreased testosterone and male impotency.", according to a study by researchers at Fred Hutchinson Cancer Research Center.
The researches point out that more research needs to be done in this area to prove conclusively that their results are valid.
However, the current body of research certainly does suggest that high levels of use on a regular basis can cause some problems that may stay with you for a very long time and at worse may just endanger your life.
My opinion on this, based on 20 odd years of following the research, is that if you have been a "wake and bake" type user since you were a teen it might be a good idea to go to your doctor, or a clinic, and ask about the symptoms of testicular cancer. If you are one of those guys who wants a large family, you might want to think about cutting down.
I'm not anti drug use, I'm not a fan of legalization either. I am all for informed choice. All plant derived substances seem to have legitimate medicinal uses. Over-use of anything causes problems. The depth and breadth of the problem(s) are impacted by your personal biology, genetics, and life experiences.