Motive, opportunity, means...those three things are all that's needed to cheat. With or without substance/alcohol abuse. I've known of situations in which peoples' inhibitions were lowered and they say and do things they wouldn't do if stone cold sober. Abusing substances does alter peoples' behavior, otherwise there wouldn't be any problems right? I've also known numerous folks who have never cheated once in their life, but drank plenty on plenty of occasions; my dad being one of those types. I'm pretty sure the only woman he had sex with his entire life was my mom. He was a one-woman-man and that's just the way he was, period.