Being faithful and monogamous is not natural for human beings. It takes work. Deep down we all know that. We have all been tempted to stray at some point or another. Even when it was only a fleeting thought and we didn't act on it. Every time we acknowledge that someone of the opposite sex is "attractive" or "sexy" we are doing nothing other than pointing out that they would be a suitable mate. Not acting on that natural impulse to want to mate with a viable mating partner requires a conscious decision. It's a constant struggle between what your body wants, and what the civilized part of your brain says you should do, in order to avoid the negative consequences of cheating on your spouse and ruining your long-term relationship. That's why affairs, and extra-marital sex, are often referred to as "a moment of weakness.