I think a lot of us are tempted sometimes.
My main reason is that I don't trust anyone to keep a secret. If I cheat, I am trusting that person for the rest of their life to never say anything. And if it gets back to DP...then I lose everything.
Maybe you're lucky and your honest reason is that you're so in love and so satisfied that you could never ben with anyone else. If so I respect that and good for you.
What's the real reason you don't cheat?