Wow.. some peoples position on this issue is quite sad IMO. Why the need to have any cheating in the first place, it's not like there is a shortage of dick or vag on this planet! When I hear people justifying this kind of behaviour it reminds me that i'm living amoungst animals :/ Sex is important but not at the expense of stabbing someone else in the back, regardless of whether you know them or not.. we're all brothers here anyway.
More importantly, why the fuck would you want to have sex with some slut douche who is willing to cheat on her partner? If she was decent she would do the right thing and COMMUNICATE with him first, tell him she's unhappy, try and resolve it.. and failing that dump him. The kind of girl (or guy) who is willing to do that to their partner is not the kind of person you want to sleep with, you may regret it too when she confronts you in a few weeks.. "hey, guess what, im pregnant.. i also have genital warts and herpes sores around my anus". I don't care how hot she is, if she is deficient in soul and moral compass then that is not someone you want to fuck either.
Lastly, have some heart for the guy. You know you would be devestated if it happened to you too. No one wants to find out they've been cheated on, you feel like a total failure. Maybe you have been lacking something but if she was decent she would have told you and not stomped on your nuts.