A guy once told me, at the beginning of our relationship, "ask me anything, I don't want to keep anything from you, ask me anything". A few months later, here was something I hadn't thought to ask him about: he told me, before we met, he had been asked to do a porn movie, and after saying yes, changed his mind and decided not to. I never knew whether or not to believe that he hadn't done it.
So, although it's a deal breaker, I cannot imagine asking a guy I've just met whether or not he's ever done porn, or even considered it.