when you say insertive vaginal sex, would this include inserting the fingers or just the penis? why would one be more susceptible to infection than the other?
When I say insertive vaginal sex, or insertive penile-vaginal sex, I am referring to a person putting their penis inside another person's vagina. If I'm referring to the risk of HIV transmission during insertive vaginal sex (and hopefully I will be more specific than that) I would be talking about the risk of transmission to the insertive partner, or the one whose penis it is.
Insertive vaginal sex does not refer to inserting fingers into the vagina. That would be considered insertive digital sex, or digital-vaginal sex (digital referring to fingers). I usually refer to it as inserting your fingers into your partner's vagina or rectum.
The reason why insertive penile-vaginal sex poses a risk for infection and insertive digital sex does not is because of the presence of mucous membranes in the penis. Anytime mucous membranes come into contact with infected fluids, transmission can happen. Mucous membranes in the penis are present in the lining of the uretrha, and among uncircumcised men, on the inner surface of the foreskin.
There is evidence that men are infected through insertive sex, but there is no evidence that people are infected through fingering.