Oral sex is not a risk for HIV. It doesn't matter if the semen was exposed to air or not - it's not a route of infection. A mouth/throat is not an anus and does not behave the same way.
The only sexual risk is having unprotected, penetrative anal or vaginal intercourse.
Anal sex is a risk because the infected fluid can enter the cell membrane through rectal mucosa which is vulnerable to trauma during penetration.
Whereas, consumption of semen is infected fluid entering the digestive system through the stomach lining / mucosa with out any possibility of rupture, also it has gastric / stomach acids that inhibits the virus.
Hence, both scenarios differ strikingly.