A mistake is forgetting to dot an (i), cross a (t), turning right when you (meant) to turn left. Cheating is not a "mistake".
Cheating is an "intentional" act.
The goal of any cheater is to hold onto all that is good in their primary relationship while addressing their other "needs" on the side. They have no interest in replacing one relationship with another or going through a messy breakup or divorce.
It's a selfish attempt to "have it all".
A cheater does not feel (sorry) for making a "mistake". When they say: "I never meant to hurt you." What they're really saying is: "I never thought you would find out." They feel sorry that (you) found out and now that everything is out in the open you hold the power in determining if your relationship will continue. Or if by chance they were to going leave you for their "side partner" they didn't want you to know there was someone else. Some people treat relationships like jobs; they won't leave one until they have another one lined up.
There's a big difference between feeling "sorry" and feeling "guilty".Nevertheless it's "intention" that determines whether an act is a "mistake" or not.
No one (accidentally) has sex!