What were they thinking?

I'm talking about big-name politicians who believe they can have extramarital affairs — or in the case of former New York Gov. Eliot Spitzer, hookups with prostitutes — and not be found out.

We've seen many political careers ruined by such stuff, from the end of Gary Hart's 1984 presidential run to the current public embarrassment of South Carolina Gov. Mark Sanford.

Half of all marriages in the United States end in divorce. And, sadly, a number of those breakups probably happen through roving eyes by one partner or the other.

So politicians, as a breed, should not be held up to any higher marital standard than any other profession, be it company CEO or doctor, housewife or construction worker, when it comes to a successful marriage.

But big-name politicians are in the public eye.

Hart, facing rumors of infidelity, even challenged reporters to follow him to see how boring his life really was.

Well, a couple of Miami Herald reporters did, and found him hanging out with a lover. The pictures of Hart on the back of a boat named Monkey Business with his mistress just made the story spicier.

But back to the original question: "What were these politicians thinking?"

Well, it is easy to say that they weren't thinking at all — hormones overpowered their brains.

But that's a cop-out.

Good politicians are planners, even schemers. They think about their public image a lot.

Not trying to psychoanalyze these folks too much, but it seems to me that getting away with something is part of the allure.

Powerful men begin to think they can do anything, that the rules don't apply to them.

And that is dangerous. Because one week it may be an affair, but the next it could be subverting some part of their public work — from bribery to double-dealing politically.

Where does one draw the line on deceit? Is it OK to lie to your spouse? OK to lie to your constituents? Lie to the media? Where does it end?

Supposedly we pick politicians to serve because we trust their judgment, rely on them to make decisions for us, in our republic form of government, because we everyday citizens don't have the time or interest to make those decisions ourselves.

It's called a "public trust" for a reason. It is both public and trust.

So, I must say, we do need to know when a public servant is not trustworthy — either in business (if he's a part-time politician with a private-sector job), or in other parts of his life, even his personal life.