Humans, like many other animals, have drives that are not in line with modern notions of fidelity. However, humans also possess the ability to reason and understand emotion. There is no difference between the desire to rob a bank because you want money, but not doing it because it is illegal, and being faithful to keep from hurting someone.
Natural selection is the ability for a male to impregnate and support as many females as possible. This is a herd mentality. Humans, as much as we deny it, are herd animals. We socialize and even move in groups. Isolation results in mental instability. Socialization is what comes most naturally for us. Modern notions of life mates are very limited in nature. There are examples, but in those examples there are typically multiple offspring per birth, which makes up for the population loss from a herd example. This is why man's nature is not for a life partner, just a suggestion by the church.
The modern idea of fidelity does not follow most research about what we know of human nature and relationships. Instead we have conformed our behavior due to religious or "cultural" expectations while our natural nature is to have several partners over our lifetime. This is evolutionary in nature since offspring from multiple partners would increase the chance of survival for a species as a whole.
We are learning more and more about the way the mind works every day. We are learning that men and women are different if fundamental cognitive ways. We are learning that monogamy isn't the rule in the animal kingdom, and though we like to think we aren't animals that we are. I suspect future societies will view our notions of fidelity as absurd.