From Scientific American:
The risk of destroying a career is nothing compared with the evolutionary drive to reproduce
An admitted affair has crumbled the career of CIA Director David Petraeus, prompting the evergreen question: Why do people with so much to lose risk it all for sex? In the last few years alone, several public figures, from former Rep. Anthony Weiner to action star and former California Gov. Arnold Schwarzenegger, have admitted to straying from their marital vows. In Petraeus' case, a miscalculation of risk may have contributed to the decision to cheat, psychologists say. “People tend to underestimate how quickly small risks mount up” because of repeated exposure to those risks, said Baruch Fischhoff, a professor of social and decision science at Carnegie Mellon University. “You do something once and you get away with it — certain things you're probably going to get away with — but you keep doing them often enough, eventually the risk gets pretty high.” Even so, men can become blind to risk at the sight of an attractive woman, and from an evolutionary perspective, cheating can be a positive mechanism for ensuring gene survival, regardless of risk, scientists say.