It might've been an accident or a hacking. It might also be his private sexual life. You're allowed to have feelings, of course, but there's technically nothing "wrong" with porn or sex. Every therapist has a private sexual life; it's not really our right to know about it. But, now you do, I'd talk to him about it...just understand that your reactions are your reactions, you have a right to them, but you didn't actually have a right to know about his sexual life...he didn't bring this into your therapy. He's not being sexually aggressive here or coersive or doing something ethically wrong. And, if you can't move past it, so be it - you move on to a different therapist.

or...maybe it's all a mistake.