Extra Marital - to be or not to be?

This is the best topic to gossip around on. Every second person I meet talks about someone or the other getting sexually involved with some other person out of the formal / marriage arrangements.

The closest animal to human is monkey. Even in most species of Monkeys, there is monogamy. Human is programmed to be in monogamous relationship(?). But I believe, human is also programed to have hetero-sexual relationship. We hear a lot of stories about cases which are actually moving away from the basic programmed way of choices... Is this evolution? If yes, where are we headed?

But when it comes to extra-marital, there have been stories of polygamy since the history is known to modern society. As per the program, all man-woman relations are considered to be for re-production. But even in monogamy, you may have re-production and if the right choices are made, genes can evolve much better. So why are we programmed as against our closest relatives (monkeys) to be polygamous? or is it just exception to normal behaviour?