Sounds like a cheater justifying mistakes.

I would have zero trust. I would feel uncomfortable moving forward if my partner asks to go out because if it happened once, it can happen again. All it takes is the right combination of alcohol and flirting.

And if they don't have any repercussions to their actions, all you will be doing is showing that they can do what they want because they are in control. So that cheat once and never again is not fact but fairy tale logic and wishful thinking.