If you’re sticking your face into fecal matter when you’re engaged in oral sex then you’re doing it wrong.
Especially with fellatio, you’re not going to be close to fecal matter in normal circumstances.
You seem to have peculiar idea about hygiene in traditional cultures. Medieval European cultures may have been adverse to bathing but this is not necessarily typical. I’ve stayed with indigenous people in Panama and elsewhere in the world and they bathe daily whenever possible.
As has been mentioned, bonobos and other primates engage in oral sex. You are proposing that at some point human ancestors abandoned this practice, only to start engaging in it again at some later time. This isn’t a parsimonious scenario.
Even if earlier humans were not concerned with cleanliness, you are projecting the modern aversion toward body odors and contact with feces/urine on other cultures. Just because you might find unwashed genitalia off-putting doesn’t mean that earlier humans would have any such concern.