You're confusing mistake with accident.
Yes, it is a mistake. You chose to do it and it was wrong, making it as mistake.
Things can be both choices and mistakes...
Well something you do in the heat of the moment is not the same as something you have thought about rationally. Not that I condone cheating, but a moment of misjudgment does not mean you haven't made a mistake.
Unless you are heavily intoxicated, but even then the mistake is in letting yourself get that wasted.
A friend of mine (who has a girlfriend) kissed a girl (who HAD a boyfriend).
It pissed me off, because the girl's boyfriend was seriously the sweetest guy, and my friend had been acting like a lovesick puppy over the girl he's been dating.