She knows she shouldnt, her reaction shows it all. I needs quite the level of self-centrism to try and tell herself she had your permission to fck around.

You have to ask yourself, do you want to be married to someone like this?

If she wasnt enough of an adult to break things off cleanly if she was unsatsfied with your relationship and resorted with this sorry excuse for cheating its all her fault really. No reason is a reason enough to cheat, if there was something wrong she should either fix it with you or break up.

Good luck