The length and intensity of the sexual act may affect transmission. If his dick is pounding against the back of your throat, your tissues may become irritated, becoming more permeable and increasing the likelihood of infection. If you have had unprotected oral sex and you have a sore throat or a dry cough, get tested for oral gonorrhea and strep. If you have sores, discharge, or burning when you pee go to your doctor or STD clinic. Some people are more susceptible to infections than others.
Some people have better oral hygiene than others.
Bleeding gums, sores in the mouth, and abscessed teeth may increase the likelihood of transmission. HIV-positive men and women may be more or less capable of infecting others, depending on how long they've been infected themselves. Avoid coming in someone's mouth. Or having them come in yours. Remember, even with pre-cum there's still some risk.