awesomeClaw said:(ALSO: If you are in a relationship, your partner would never find out.)
And herein lie the two flaws of the question.
#1- The aggro for the unfaithful transcends space & time. It nevers expires or reaches a statute of limitations.
#2- You will still know that you cheated. If you are the kind of person that could carry off that deception for the rest of your life... better to just tell your partner and let them leave you for someone more deserving.
and....
#3 If you're not in a relationship, or a relationship with a mutual & explicit expectation of faithfulness, then go for it! Life is too short. Consensual sex is one of the most enjoyable parts of the human experience. Remember to insist on safe sex practices.