I don't really have sex with people I don't have some level of trust with, but we are all sleeping around a bit and even someone you trust can make mistakes. Most STIs are curable or not a big deal, but HIV changes your entire life. With PrEP, you allow yourself some freedom to take a calculated risk. I mostly don't have sex with someone unless they are on PrEP too, because I only have sex with responsible mature people and in our community, taking PrEP is the responsible and mature thing to do. But we are still fooling around with a lot of people. If I am on PrEP and the person I am hooking up with is on PrEP, the chances of getting a life changing STI become extremely low.
There is another factor. With my doctor's office, which sees a lot of people in the community, they won't let you be on PrEP unless you get tested every three months. So in many cases if someone is on PrEP they are also getting regular testing done. That is not a direct result of PrEP, just an administrative side effect, but what it means is that if I go to a sex party and someone wants to go down on me, as happened in January, then if we are both on PrEP and getting tested every three months, then that interaction is very safe. When I did this it was with someone my girlfriend (who was also at the party with me) knew before I did, so there was some level of community trust there.
Hope that is a helpful answer.