These kinds of questions bring morality, social construct, and other abstract thoughts into the picture. There are people like Vapid who believe the way they do, and people like Riker Slade who believe the way they do. Neither party is necessarily wrong, as it comes to one's beliefs or lack thereof. We can only answer for ourselves, and even then, under these perfect circumstances, like Blackwatch initially said, it's a 100% fact some of you are lying. It's typical for humans to think they would act a certain way in any given scenario, yet when reality hits and the situation arrives, you behave completely opposite. That is one of the fatal flaws of humans, our very humanity. Bearing that in mind, here are my thoughts:
I hold myself to a higher standard than a lot of society does in this day and age. Much like my favorite characters Naruto, Goku, Aang, Yugi, and Superman, I try to uphold righteousness and exude those traits I find admirable in said characters. Things like honesty, integrity, truth, and fairness matter to me as a person and are defining characteristics of who I am. Being a cheater means being a liar, a betrayer. That's not something I would ever do to somebody who I cherished. What's more, when we were in our prime as a couple, I truly only had eyes for my lover. She was the only thing I wanted in life and no other female caught my attention in the slightest. Call me old fashioned, but I feel that's the way it should be and that's the kind of love and devotion I hold inside me. I love on a 10, if you will. That relationship deteriorated for a lot of reasons outside of that though and the emotional immaturity of my chosen partner was pivotal in that.
However, people who say "Just leave the person if you want to cheat" are either very inexperienced in the dating world or have yet to be truly bonded to another human. Sometimes, even when you want to leave, you just can't. There comes a point when that person becomes a part of you. Like what Owen Wilson said in Wedding Crashers, "True love is your soul's recognition of its counter-point in another." Temptation will always be there for some people and there are moments of weakness, however inexcusable one feels they are. It's not exactly unheard of for people to find comfort in the arms of another after extensive problems when someone they truly love or to be "taken advantage of" emotionally in that sense.
TL;DR - I personally would never cheat because I couldn't live with the person that would make me, but not everybody holds themselves to that standard or is able to ward off basic human thought. Men are hard wired to crave *** and not every male can fend off this biological imprint. The female psyche can be extremely fragile and is easily swayed in times of distress. And to quote Chris Brown, "These hoes ain't loyal".