Why does this continue to happen? Smart people doing dumb things.
She was the valedictorian of her high school class, as well as homecoming queen; a fitness champion at West Point; and has a graduate degree from Harvard.
He was in the top 5 percent of his graduating class at West Point; was the General George C. Marshall Award winner as the top graduate of the U.S. Army Command and General Staff College class of 1983; earned an M.P.A. in 1985 and a Ph.D. degree in International Relations in 1987 from the Woodrow Wilson School of Public and International Affairs at Princeton. Shall I go on? No, need. You know the rest. A chest full of ribbons, and held in such high esteem, some in GOP circles wanted him to run as Romney's veep candidate.
Of course, he can redeem himself after lying low for a while (unless Benghazi ensnares him). Look at Eliot Spitzer and Bill Clinton. Nevertheless, the numerous and repeated episodes of sexual improprieties by "responsible" members of society remind us that under the skin, we're not much different from "the way we were" a million years ago.
Powerful Men + Attractive Women = Fireworks
Not to say that the converse coupling doesn't sometimes occur. However, the predominant evolutionary impulses tend to get their way.
Suggested reading: "The Sex Contract: The Evolution of Human Behavior" by Helen Fisher