If there's something I've learned to be a FACT in this life, it is 'What goes around, comes around' or Karma. Whichever way you prefer to call it.
No matter how 'Foolproof' someone may think 'cheating' is, you've broken a trust and penance will be paid is some way.
Besides, if I'm looking to cheat then that means there's already something wrong with the relationship...I'd END IT first, then go about my life. It's what's best for me, it's what's best for her.