Saliva isn't the problem. Mucous membrane is.
Chlamydia, Gonorrhea and Syphilis can ALL be transmitted through ANY mucous membrane (the mouth is just ONE area -
guess where the other areas are?) In fact, condom use while reducing the instance of contracting Syphilis does not ELIMINATE the risk (as condom use with Gonorrhea and Chlamydia does to a greater extent).
Then then are the 'viral' diseases...
Bottom line: You stick an infected penis in a mucous membrane without protection, you are risking infection...