I would say that using marijuana does not mean it
will lead you into harsher drugs. For some, yes. But those people would likely go that path anyways. I know many people in my age bracket that never progressed beyond a joint or two, here and there.
Long term marijuana usage can lead to addiction. Even then, the usage has to be considered compulsive and abusive - where using the drug interferes with daily functioning (in regards to family, work, school, friends, etc.).
A good article to read is this:
InfoFacts - Marijuana
The following article does have some information on lowering the sperm count in males:
Marijuana: Health Effects