Risk of HIV transmission from semen to vagina is supposed to be because of vaginal mucus membrane absorbing the virus. Mouth surface is also mucus membrane. So why risk of semen-to-mouth transmission is less than semen-to-vagina?

because by the nature of the enzymes in the mouth the likelihood is diminished. Oral enzymes which include saliva and agastric reflux enzyme appears to be hard on the virus than those of the vagina.

And I believe that the thrusting nature of vaginal intercourse can also cause minute tearing and damage to the vagina allowing easier transmission. Correct me if i am wrong here though.