If you can bag one hot chick, you can bag lots of hot chicks. Sure, if you're kinda ugly, a 6-7/10 average dude - you're thinking 'Wow that girl is amazing - why would anyone cheat on her ever!?!'
But if you're a guy who dates that girl, you can date lots of other hot girls.
Couldn't have said it better myself. There is no justification to cheat. Anything you tell yourself is a "justification" to cheat is just a reason to end the relationship.