You’re right, it’s quite rare to get or to give gonorrhea or chlamydia of the throat while performing oral sex on a woman. When present, these bacteria tend to live in the back of the throat and in the cervix, high inside women’s vaginas. These are areas that just don’t get a lot of contact with each other during oral sex on women. (Transmission can happen more often for those who perform oral sex on men, since bacteria living in the tip of a penis can be in contact with the back of the throat this way.) We don’t usually do throat swabs on people who perform oral sex on women as part of a regular STI screen.
Hope that helps answer your question.