It's not particularly about trust or not trusting someone, it's about my responsibility to all the people I am having sex with including private life too.
I have also thought about what if my partner was tempted, it's not completely out of the question with anyone no matter how long you have known them and in that case because my safety then affects that of my clients, so I would rather be safer than risk passing anything on even if it's only a tiny chance. And same vice versa.