My RABF did some horrible things to me when he was high...sometimes I wonder why I even still talk to him (working on those motives in recovery), but he never once cheated, and always has been adamant about not being a cheater. I know I wouldn't be around today if he had.
Addiction is no excuse for ANY nasty behavior, but cheating is kind of the ultimate wrong IMO. I personally can stick by someone who has not treated me the way I deserved to be treated in the past due to addiction, only if that person makes a 100% effort to make amends to me and heal themselves/change such behavior, but cheating is my limit. I could never forgive that! Guess it comes down to individual limits and tendencies on both ends...