Politicians, by their very nature are psychologically dysfunctional individuals. They need constant adoration from the voters while simultaneously needing to control people, things, and money. I'm sure this megalomania probably influences their personal lives as well. It is not uncommon to hear of politicians that get involved in extramarital affairs of one sort or another. In fact if truth be known, you'd probably find that virtually all politicians have some extramarital adventures in their closet, or some other (mostly dominant) sexual kink. I think it is probable that most have some severe control issues at a minimum. They are probably all ripe for a long stint on a psychiatrist's couch to be honest. Along the same vein, most all of them have addictive personalities, in which they become addicted to booze, gambling, drugs, sex, or more than one.
So to find that yet another politician has found himself in a sexual imbroglio is really not all that surprising, or even disturbing to me. Hell, I'm frankly surprised it was straight consensual heterosexual sex without the promise of remuneration.
The surprise is in how he handled it. Most politicians, when confronted with a blackmail attempt will capitulate to the demands of the blackmailer because they NEED to stay in power and such exposure is usually not compatible with that. The fact that he came forward and admitted it does at least say something about the way he approaches his moral failings.
Now, let us be clear here. One of the things that sets Conservatives apart from Liberals is a moral code. Many Liberals subscribe to moral relativism such that nothing is absolutely right and nothing is absolutely wrong, everything is a shade of gray. I would suggest that conservatives should be given the credit that at least they have morals that the subscribe to, even if they do not always abide by them. They at least have a code by which they judge themselves. If you have no absolute standard of morality, you cannot judge yourself against it. Everything is relative.
UPDATE: One other point that has come to light. Apparently the affair occurred during a time when Ensign and his wife were legally separated. So as far as violating his marriage vows go, that is debatable. Under the law in most states, when you are filing for divorce, you may not claim infidelity as a reason for divorce for any affairs that occur after the couple has legally separated. The law recognizes that the marriage vows are no longer operative at that point. Now, he should have known better to get involved with a woman who was still married, but she chose to violate her vows, he apparently did not.