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Thanks for your great question.
I see what you mean about the oral sex chart. The main difference in risk I think you’re referring to is with chlamydia and gonorrhea related to mouth on penis sex. As it says on the chart that gonorrhea, chlamydia, HPV, herpes and syphilis are easily passed (when someone has the infection) during mouth on penis sex without a barrier versus HPV, herpes and syphilis being easily passed during mouth on vagina (probably more like vulva) sex without a barrier; whereas chlamydia and gonorrhea are not commonly passed for mouth on vagina (or vulva) sex.
The chart is a pretty general, simplified tool so I can see how it might not help to explain the nuances of the different types of sex in the different areas we can have sex. It might be helpful to think of how STIs (sexually transmitted infections) are passed; for the STIs mentioned above, that can be skin-to-skin contact (syphilis, HPV and herpes) and sharing of fluids (semen, vaginal fluids).
Both of these kinds of oral sex involve skin-to-skin contact. The chart shows that with skin-to-skin contact, syphilis HPV and herpes are easily passed. The transmission of chlamydia and gonorrhea is possible for both mouth on penis or mouth on vulva sex however; the chances are greater for mouth on penis sex. This would be because there is a greater chance of semen rather than vaginal secretions being in contact with that area, or more specifically, the throat. There tends to be more direct, deeper contact between the penis and throat versus the vagina or vaginal secretions and the throat. Mouth to penis sex also tends to be more vigorous in terms of contact inside the mouth and throat.
And to answer the other part of your question; which way is the infection being passed? Well, it can be both ways. But again, the chart is meant to be a simple tool so it doesn’t capture the difference in risk to giver or receiver. It is possible to be infected with chlamydia or gonorrhea in the throat. The chart is describing the type of sex (mouth on penis/vulva/vagina), and not exactly where the infection would occur (for instance, throat or urethra).
On the chart, you can also see that using a barrier significantly reduces your chance of giving or receiving any infections for both kinds of sex.
Thanks again for your question.
This answer was posted on October 27, 2016