Cheating....can it still work?

Can a relationship still work if someone or you have cheated and you told your bf or gf?

Would you choose to still stay in the relationship if your bf/gf told you that they had cheated on you? or would you walk away?

I don't understand people who cheat...if they have the most awesome person as their significant other, why do they still cheat?
For example:
This guy I know, he has this gf who is so nice and pretty and really smart and loving to him, yet he still cheated on her. He told her about it, and yet she is still with him.

Hmmm.....mindboggling

I've just realised that there is a theme to all my journals...Cheating :P