If you’re trying to be a daddy you might want to lay off the weed:

A study of 22 college students who were heavy marijuana users suggested their sperm moved too fast too soon and lost oomph before they could reach their destination � the egg.
“Sperm in semen are supposed to sit quietly and wait for their chance to get out of the semen,” said Dr. Lani Burkman, a researcher at the University of Buffalo. “The sperm of marijuana smokers when sitting in the semen were very, very active.”

The men studied smoked marijuana about twice a day.
Burkman said the effect on the sperm came from pot’s ingredients. She said earlier laboratory work indicates sperm generally can’t keep up an accelerated pace for more than six hours or so, while it may take 24 hours or longer to get to the uterus.
If the sperm swim vigorously early on, “they will not have this vigorous motility by the time they are getting somewhere close to the egg and will not be able to penetrate the egg,” Burkman said.
The research also found that the marijuana smokers had only about half as many sperm per volume of semen as the control group of men who did not smoke marijuana.
Burkman said it can take four to six months of marijuana abstinence for the sperm characteristics to return to normal.

There are probably a bunch of other ramifications that aren’t discussed in the short blurbs popping up in the press.