hey lnm - i get that - the not being able to be with someone who betrayed you like that. i know oh everyone says that mentality but i'm literally the same. i think for some of us it is a very simple black and white issue if you like. i think i could possibly forgive someone sleeping with someone, realising what they'd done and immediately coming to me and saying jesus i fucked up so fucking terribly and i am so bloody sorry etc etc. it is possible i could rant and rage and calm down and forgive them an idiotic mistake - because we're all capable of idiotic states and being momentarily selfish, self absorbed, childish and whatever.
what i know i could never forgive is someone lying and deceiving and doing something behind my back without telling me. i couldn't forgive them the disrespect and deceit and i could never forgive them the cowardice (re: i could never ever respect them as a grown up again).
it's just part of who we are and not even a decision really. what is incredible is that he didn't know about you. you didn't know he was capable of this and he utterly underestimated who you are in your values and self respect and core of who you are.
i cannot imagine the disrespect and utter lack of care involved in sneaking around behind the person who you're meant to be sharing a life with's back and not being overcome by the need to be honest. i literally can't imagine EVER being able to do that. so why on earth would i want to be with someone who could do that?
sorry long tangent of ME! 
i have had chicken drama and weird work quandries today so possibly overly in need of distraction hence venting all this at you. i hopeyou've had a good day.