When you get into a relationship and you're at the point you're going to have sex with your partner, do you want both of you to get tested first or do you ask the person if he/she is clean and that'll be enough?
I myself would get tested first before doing so, you can't see what they have on the outside and the person might not know they have something either.
I don't see it as not trusting the partner but rather precaution.
What about you?