From your perspective, he's cheating. From his perspective, he's not. As far as he's concerned, it's no more cheating than if you were dating and broke up. There's just a little bothersome paperwork in this case.
And nobody else's opinion really matters.
My question for you is: if things do work out and you reconcile, will you ever be able to forgive him for cheating in your eyes?
M:23 T:26 Me:53, Wife: 60 S:18 D:16 filed 7/16 W moved out 4/28/17