I'll be honest.
I cheated because I was with the wrong person at the time.
I was in love with a man that circumstances didn't allow me to date. So, instead, I dated a laundry list of guys that I liked but didn't really care about... and whenever he came into town (he travelled a lot), I would cheat on bf-of-the-month with him.
I'm not proud of it, but it taught me a lesson.... don't settle for someone you can't completely give your heart to. I hurt all of those guys because I was looking for someone to pass the time inbetween his visits.
(oh, and as it turns out... he wasn't travelling... he was MARRIED. Talk about karma.

)