I think the missing link is the fact that the male urethra ends at the head of the penis, so oral contact with a penis can cause urethral infections to that male. Whoever is giving the oral isn't themself at risk of urethral infection, of course. And I believe you are correct that a female is a higher risk of that kind of infection if her partner provides ass-to-mouth followed by vaginal contact (as there are more organs which could be affected).

VV