Well, 19 years in and I haven't cheated yet.
Personally, I don't think anybody can truthfully say "I would never cheat on my partner".
I think there are 3 types of people in the world.
Ones who are happy to cheat and lie and don't feel bad about it. - This is my Dad. He's a knobhead, so lets move on.
Ones who know they are capable of it, but do their best to ensure it doesn't happen. - This is me. I'm probably not really all that monogamous. I don't in theory see anything wrong with sleeping with multiple people. I don't really see anything wrong with cheating as an act either.
What is wrong however is hurting your partner. My DP would be very very hurt if I cheated on her. Now, I could get around that by having her never find out. Except that I'm not stupid enough to believe that the perfect crime exists. I could be halfway around the planet cheating with someone with no connection to be or DP, and there's still a chance she'd find out somehow. Maybe I'd get an STI, maybe my phone would accidentally butt-dial her while I was in the act. Who knows. But that's what stops me cheating, not the "This is wrong, I shouldn't do it", but the "This has a chance of hurting DP, so I am unwilling to do it."
The final type of people are the ones who say "I would never cheat" and truly believe it. Whether for religious reasons, moral ones, whatever, they truly believe they're incapable of cheating. This was DP, right up until the moment she found herself teetering on the cliff-edge of an affair. She wasn't even knowingly unhappy in our relationship at the time, someone just paid her the right kind of attention at a time she was feeling particularly emotionally vulnerable and suddenly she had one foot over the abyss.
I reckon this last category, the ones who "think they could never", probably end up being less faithful overall than the ones who know they could but try not to.