Do human instincts lead us into monogamy or is it just the opposite?

Recently I’ve seen a few tv shows that did studies on humans and monogamy. They claimed that our instincts don’t automatically make us desire to be with one person forever (at least not sexually). Do you believe this theory? Are we trying to force something that just isn’t programed in us? Will we always desire to be with other men/women even when we should be happy with the person we already have?