Oh god, what a horrible situation for you. For all of you in Xxxx's family.
But I have to wonder, did he 'cheat', did he actually commit adultery? Or did he get carried away in a mindless grope? I'm not saying that you should ignore it, but that maybe it's more like "OMG, I shoplifted, I don't know what came over me, I feel awful!" rather than "Well, I've been shoplifting for the last year or so. I'm so sorry."
Have you really been betrayed? Or has he done something disgusting and shameful, but which will never happen again, and for which he and your db have been hurting over and suffering 'punishment' for already?
I do hope I don't hurt you more by this post. I truly don't mean to belittle your distress at all, but I'm concerned that this will escalate too far, and things will get worse.
Don't go away from each other. Things won't blow over - you must talk to each other, and listen to each other too.