I've done some really low down cheating...I fucked my best friend's (a guy who has literally saved my life and put himself at extreme risk of death or catastrophic injury in combat with me while deployed) girlfriend behind his back. :/ Not cool.
I've fucked my ex girlfriend's girlfriend (while we where seeing each other still...polyamorous relationship) behind her back...but because it was polyamorous and because we both came clean to her together and stuff, she forgave that incident pretty well.
I was banging my Ms.Gallium behind her Ex BF's back...but, like, fuck that guy anyway. Plus, this was a bit different because I *loved* her and she me, and it was not just trying to get a nut off...
Blah, I've been cheated on many times..like it hurts, it stings, but to be perfectly fucking honest, it never devastated me, and I've never ended a relationship over it. I'd probably only do that it was particularly egregious, like if she was withholding from me in preference of whoever, or if she was unsafe about it (both STDwise or otherwise, like dangerous situations or stuff like asphyxia play BDSM scenes) or if she did it frequently and repeatedly such that I was getting hurt too much/too often vs the amount of pleasure I otherwise derived from the relationship.