How do you explain your 5 year old why you separated from her mother without bringing up the real issue?

I have a five year old daughter which is asking me why I left her mom and married someone else, She likes my wife which is a really weird question coming from a 5 year old. She caught me by surprise and I really don't know what to answer. Her mother and I broke up even before she was born, obviously she was there. She was cheating on me with someone who was really close to me. Anyways, how do you tell a five year old why her mother and I broke up?