QUICK BIOLOGY LESSON.....
Chlamydia, human papillomavirus (HPV), gonorrhea, herpes, hepatitis (multiple strains), and other sexually transmitted diseases (STDs)—including HIV—can be transmitted through oral sex. While the exact risk of transmitting HIV through oral sex is unknown, it is generally tough to be lower than other sex practices. Any kind of direct contact with body fluids of a person infected with HIV (the virus that causes AIDS) poses a risk of infection. The risk from most of these types of infection, however, is generally considered far less than that associated with vaginal or anal sex due to the membranes in the mouth being considerably THICKER than that of the vagina/anus.

If the receiving partner has wounds or open sores on their genitals, or if the giving partner has wounds or open sores on or in their mouth, or bleeding gums, this poses an increased risk of STD transmission. Brushing the teeth, flossing, undergoing dental work, or eating crunchy foods such as chips relatively soon before or after giving oral sex can also increase the risk of transmission, because all of these activities can cause small scratches in the lining of the mouth.
AND....
Although a common misconception,[16] oral sex (by means of fellatio) alone cannot result in pregnancy

Zara Davies