If you think there's never been a case of someone getting HIV through oral, you're sadly mistaken. It does carry some risk, but it's extremely low. Cuts or sores in the mouth and/or cuts or sores on the penis increase the risk.
The reason people say it's nonexistent is because it's so low as to be VIRTUALLY non-existent. For gay males, the risk increases some because more gay men (In America and Europe) have HIV, so obviously the risk increases. But even then it's still so low that worrying about it would be tantamount to worrying about lightning hitting you on the way to the mail box.
Recieving Oral Sex, to my knowlege is not a concern, but giving can have some risks, no one will ever tell you that the stomach will destroy all the virus, the virus can not live well in the stomach but there are no guarantees when giving oral. When anyone ejaculates in someone's mouth, there is relatively a lot of fluid, a lot more than when you kiss someone for example, why kissing is not a concern. Here the fluid could of gone everywhere in his mouth, if he had bad gums, sores, or the stomach just didnt absorb it all. Who knows, its only one case of a few that you hear about when giving oral sex. I ve seen some of the Dr's post and they themselves consider giving a low risk, but not a no risk at all situation.
Oral sex carries some degree of risk depending the situatuion, receiving oral sex carries a very small teorical risk (being the insertive partner), but giving oral sex carries the double of risk the CDC says that the risk per act of receiving oral sex is 1/20000 and for giving oral sex 1/10000 this risks can be increased if the insertive partener eyaculates in the mouth of the receptive partener, and if the recptive partener has bleeding gums, open sores in the mouth, ect. The case ja246 posted is from a gay person involved is several unprotecetd oral sex like he said, and anal sex, his partner was gay, and unfortunally gays have a high prevalence of hiv, so his risk would have been bigger, his partner also had other STD that undoubtly increased the risk.
This is a post from a good page on studies made of oral sex.
http://hivinsite.ucsf.edu/InSite?page=kb-07-02-02#S3.4X
The doctors will tell you that the risk is minimal at best and statieg that giving is probably more risky than receiving albeit the small risk in both. Believe me, the only case I have ever come across after extensive research on the internet is this one below from a story by Richard. Notehowever he does mention a sore on penis at the same time as seroconverting which arguably questions his true HIV exposure?
http://www.avert.org/manstory.htm
Another great website, along with this is www.thebody.com. Again, a credible Doctor Bob will quote the same level of risk low to negligible
There has not ever been a case of HIV being transmitted through oral sex. Other STD's can be transmitted by oral sex though.