The only thing that separates us from the animals is reasoning - which is why we do not usually kill each other over something trivial, like, you have a twinky that I want, I brick you death, I have the Twinky
However, we have been conditioned that this sort of behaviour is unacceptable, we won't get away with it and will be punished for it.
Same goes for infidelity - it is "easier" stray if you come from a culture, or upbringing where monogamy was eroded. The opposite is true of those who hold monogomy as a virtue and will not break it no matter what temptations there are.
I have been monogamous for the past 8 years - one woman. I have been tempted, but never indulged.
Would I kill you for a Twinky? If I could get away it, too right.