It looked like a mod asked me for proof :D Their asking for proof comment is gone, but I decided it's still a good idea.
This makes it sound like the sperm are maybe even stoned and swim weird, so I may have been right after all, just not about it killing sperm. But still only the sperm that are best at swimming stoned would make it :D
Men who smoke marijuana frequently have significantly less seminal fluid, a lower total sperm count and their sperm behave abnormally, all of which may affect fertility adversely, a new study in reproductive physiology at the University at Buffalo has shown.
This study is the first to assess marijuana's effects on specific swimming behavior of sperm from marijuana smokers and to compare the results with sperm from men with confirmed fertility. Marijuana contains the cannabinoid drug THC (tetrahydrocannabinol), which is its primary psychoactive chemical
http://alcoholism.about.com/cs/pot/a/blub031013.htm
And this study seems to agree with me
Scientists at Buffalo University found regular smokers had significantly less seminal fluid, and a lower sperm count.
Their sperm were also more likely to swim too fast too early, leading to burn-out before they reach the egg.
Lead researcher Dr Lani Burkman said: "The bottom line is, the active ingredients in marijuana are doing something to sperm."
http://news.bbc.co.uk/2/hi/health/3186686.stm
This one says I was right about it killing sperm directly, and it says it is "temporary" because they have receptors for marijuana. Seems I was also right about genetic mutation
Substance Abuse. Cocaine or heavy marijuana use appears to temporarily reduce the number and quality of sperm by as much as 50%. Sperm actually have receptors for certain compounds in marijuana that resemble natural substances and which may impair the sperm's ability to swim and may also inhibit their ability to penetrate the egg. (Alcohol itself does not appear to affect fertility unless it is so abused that it causes liver changes.)
Smoking. Smoking impairs sperm motility, reduces sperm lifespan, and
may cause genetic changes that affect the offspring. Additionally, a 1999 study found that men who smoke have lower sex drives and less frequent sex.
http://answers.yahoo.com/question/index?qid=20060927160139AAkrTjw