Even though you might think that it is not part of the actual intercourse, you always expect or at least appreciate an oral foreplay, but you do not ever think that it might catch an infection just as easily as the other mucous membranes in the body. We tend to ignore that it can get sick, just like the vagina or the penis, but due to its surface and because it is in connection with sexual organs, it can get almost any sexually transmitted diseases. HPV virus might not give you warts in your mouth, but it can lead to throat cancer. Fungal infections can also spread in the mouth, perhaps some bacterial ones can be defeated by saliva as they enter the oral cavity.
Even though you never see it in porn that a man uses a condom during oral foreplay, and it is even rarer to know what to use if a woman is pleased orally, dental dams should be worn, which are sold at surprisingly few spots, but if you roll out a condom, cut the ends and make a rectangle of it, it is an effective protection during oral sex, so you can be sure that you have done everything for the oral hygiene.
Still, brushing teeth is essential after sex, especially in case of women who swallow the man's semen. If it was infected, then there is not much they can do to protect themselves after all, but cleaning the rest of it out of the oral cavity is still better than leaving everything inside.