This is an important question, but very little research has been done. With the exception of herpes1 virus, there are no reliable transmission statistics for oral sex. We do know that without condoms, oral sex is less risky than vaginal sex, and anal sex is the highest risk behavior for STI transmission.
Oral Sex on Men
The person at greatest risk is the individual who is performing oral sex on a man. There is more risk if you get semen in your mouth. You should not perform oral sex if you have sores in your mouth, burns or bleeding gums.
HIV risk is very, very low for the person performing and extremely low for the person getting the blow job. Chlamydia and gonorrhea can infect the throat, but it is much less common than genital infections. The risk of getting chlamydia from receiving a blow job is very small. The risk of getting herpes2 in your mouth is extremely small.
A man who does not already have oral herpes1 (by age 50, 80% of people have herpes1) can get genital herpes1 from receiving a blow job. If you already have the herpes virus in your body this provides a significant immunity against acquiring the virus genitally.
Oral Sex on Women
Oral sex on women is much lower risk for both partners. Menstrual blood and vaginal secretions can contain HIV. But, there have been no confirmed cases of women transmitting HIV through oral sex. HPV infection of the throat leading to throat cancer is possible, but rare. Gonorrhea and chlamydia transmission are highly unlikely.
A woman who does not already have oral herpes1 (by age 50, 80% of people have herpes1) can get herpes1 genitally from receiving oral sex. This is actually one of the leading causes of new genital herpes1 infections. If you already have the herpes virus in your body this provides a significant immunity against acquiring the virus genitally.
The best way to protect yourself is to use condoms and dental dams for oral sex. Using plastic wrap is a controversial alternative to dental dams or condoms. One research study has shown that the herpes virus can not pass through Glad brand plastic wrap. No studies have been done to evaluate the effectiveness of plastic wrap in preventing HIV or other STI transmission. Scientists believe that using plastic wrap may be better than nothing, but latex barriers are preferable.
Having multiple sex partners is inherently risky. Deciding how to balance risks and benefits is something that should be discussed and weighed with your partners. For many people, the pleasure of tasting and feeling their partners without barriers is worth the small risk of STI transmission. Know your STI status by testing regularly. Playing with partners who test regularly, do not use IV drugs and use barriers for intercourse can help to limit risk.
I hope this article will help you to assess your risks and discuss the use of oral barriers with your partners. Planned Parenthood and the CDC are excellent sources of information about STI's and prevention.