Specific micro-organisms inhabit the mouth, just as they inhabit other body orifices (openings). They rarely cause problems within their natural ecology. If they are transmitted to other orifices, they can cause infection. One typical example is a harmless bacteria of the mouth which can come in contact with the penis. The germs enter the urinary tract, and cause male UTI.
The membranes which line the mouth are naturally subjected to tiny lesions. It has been estimated that there is gum bleeding after brushing the teeth in at least one-third of any given population. Small ulcers can be present at the sides of the mouth. The tongue can be sore for a variety of reasons. All these factors can make the mouth an "unsafe" place for sex.
Diseases known to be transmitted by oro-genital infection are: the herpes virus cold sore, yeast infections, AIDS, gonorrhoea of the throat, and syphilis chancre of the lips. At least two cases of AIDS have been contracted this way. It would seem unlikely that a woman would wish to kiss a partner with a sore on the mouth, or that she would perform oral sex on a penis with a "drip". Yet all infections have an incubation period. There is a time lapse between contracting a disease and the appearance of symptoms. Incubation periods vary widely with different STDs; they can take years for AIDS. With a new partner, the incubation period must be taken into account.
In some cases, both partners are asymptomatic. There are no signs of disease to remind lovers that oral sex can be hazardous. Avoid direct mouth contact with semen. Where there is high risk sexual activity, one option is to completely avoid oro-genital sex. If this is unacceptable, wait until a new partner has been tested and is known to be infection-free.