As far as I know, Prep does not protect against Gonorrhea or Syphilis, and there is some evidence that users (in a study of gay men) of Prep have slightly higher incidences of other sti's as they perhaps took more risks believing that they were "safe"
I guess, the faulty thinking here might be "Well HIV could be really serious (true) but the others aren't (not true)
I've said it many times (as have others) - I punt on the basis that everyone is doing bb with someone and I can't control that. So, I take the risks I'm prepared to take (Owo, deep throat and rimming) and get tested regularly.