The National Organization for the Reform of Marijuana Laws (NORML) stated on its website (accessed Mar. 17, 2006):

Government experts concede that pot has no permanent effect on the male or female reproductive systems…

A couple of lab studies indicated that very heavy marijuana smoking might lower sperm counts.  However, surveys of chronic smokers have turned up no indication of infertility or other abnormalities.

Mar. 17, 2006