There is an earlier thread on this board that begins with the post: New Hobbyist wanting feedback...6/18

As this thread developed it evolved into a debate of sorts as to the merits of spitting vs swallowing when engaged in oral sex. The choice of rather to spit or swallow is not really relevant if the notion is that one vs the other is going to somehow decrease/eliminate the risk of contracting an STD such as HIV/Aids or virtually any other STD for that matter. Once semen comes in contact with the mouth the RISK of contracting an STD has been realized whether the recipient chooses to spit or swallow. Stomach acid and its detremental effects on microorganisms does NOT diminish risk of acquiring an STD in any measurable way. I am sure there are any number of opinions in the general public about this issue and I am sure it is a topic that has been discussed here and elsewhere many times.
However opinions and facts do NOT always match up.

HIV/Aids can and will have lifelong consequences for those who
acquire it by whatever means. Therefore for your own health/safety and those that you may be intimate with please rely on reputable informed sources when trying to educate yourself about STD's and the risks of acquiring them.

There is an excellent CDC report dated December 2000 entitled:

Preventing the Sexual Transmission of HIV,
the Virus that Causes AIDS
What You Should Know about Oral Sex

A few excerpts from that report follow:

"Oral Sex Is Not Considered Safe Sex
Like all sexual activity, oral sex carries some risk, particularly when one partner or the other is known to be
infected with HIV, when either partner’s HIV status is not known, and/or when one or the other partner is not
monogamous or injects drugs. Numerous studies have demonstrated that oral sex can result in the
transmission of HIV and other sexually transmitted diseases (STDs)."

"The risk of HIV transmission from an infected partner through oral sex is much smaller than the risk of HIV
transmission from anal or vaginal sex. Because of this, measuring the exact risk of HIV transmission as a result
of oral sex is very difficult. In addition, since most sexually active individuals practice oral sex in addition to other
forms of sex, such as vaginal and/or anal sex, when transmission occurs, it is difficult to determine whether or not
it occurred as a result of oral sex or other more risky sexual activities."

This CDC report in its entirety can be found through the link below.
Scroll down the page and click on the heading "Specific Diseases".
Click on HIV/AIDS and then click on Contracting HIV.
Finally click on:
Preventing the Sexual Transmission of HIV, the Virus that Causes AIDS: What You Should Know about Oral Sex - CDC

You will need Acrobat reader installed on your computer to
be able to read the document since it is in .pdf format.

This document is also available at the CDC website:
http://www.cdc.gov/health/default.htm

I provided the link below because it is such an informative site
with respect to STD's and many other health issues.

Sorry to be so lengthy but as a health care professional I find it disturbing when ill informed information, no matter how well meaning, is tossed out there as if it were factual.

And HIV is not the only worry. Statistics say 20-25% of the public has genital herpes and 80-90% have oral herpes. They may not be active this second (or are they? how closely do you look? And do you know what to look for?), but what about tomorrow? Those are just two of the more permanent stds out there, there are plenty more than range from minor annoyance to a real pain in the ass.