The reason why I love following DJ Mo Twister on twitter is because of his tweets about interesting reads. Here's one that caught my attention.
And so if you're too lazy to read, the gist of the article is about the Coolidge Effect. What is it? It's described by wikipedia as a phenomenon—seen in nearly every mammalian species in which it has been tested—whereby males (and to a lesser extent, females) exhibit continuous high sexual performance given the introduction of new receptive partners. Basically, it explains why new couples tend to more "lovey dovey" than those that have been in the same monogamous relationship for years.
According to the link, Dopamine is the neurochemical that urges us to "want it". Whenever were stuffed with dinner, its Dopamine that wants us to take that slice of cake and in sexual relationships, its the same neurochemical that surges in when we're presented with a new partner. "Dopamine surging in your reward circuitry can override your feelings of satiety, regardless of what your rational brain may think about overeating or infidelity.", according to the article.
So basically, my theory about infidelity being naturally part of being human is supported by this article. That, I guess at one point, we do get that tendency to cheat or can be cheated on cause its in our nature. I'm not saying that I encourage infidelity but knowing that this is normal, maybe those who are prone to feeling such effect should make an effort to strengthen their moral code or discipline, so they wouldn't be able to act when the dopamine urges strike and its not because of their current partner.