I had a client who told me after I had given him oral uncovered that he had been with a men. I no longer offer OWO. It's wrong of me to make assumptions about him possibly carrying something due to being with men I just don't want to risk it. I stupidly believed that because he was seeing me he wasn't into men . I felt like kicking him where the sun doesn't shine!
Yes, that was a big assumption laid on top of a bit of prejudice
If you change what you're prepared to do based on what they tell you, they often won't tell you. In general, if you work on the basis that they're all having unprotected sex with someone, and act accordingly, you'll stop having to guess if they are or not.
What 'accordingly' means will vary from person to person. Some will still be happy to do OWO and some won't, for example. Both are fine, because everyone has their own position on what risks are ok to take.