On a moral issue, what he did was bad enough. He cheated on his wife. But, morality issues are debatable. What is moral for one person may not be moral for another. It is subjective.
There are many objective reasons for his resignation.
- He broke an oath. He swore "to forsake all others" when he got married. If he breaks his marriage vows, what other oaths will he break?
- He misappropriated state funds. He used his state provided vehicle to drive to his "liaison" with his mistress (or at least to the airport). If he did it this once, he probably did it before (people rarely get caught the first time they do something wrong).
- He put valuable state resources at considerable risk. He is the head of the state of South Carolina. As such, he is a very important resource. By travelling to another country, unescorted by any security, he put himself, and the state of South Carolina, at grave risk. As a private citizen, I could do what he did and only be at risk to the wrath of my wife. But elected officials are not private citizens!
- Dereliction of duty. In South Carolina, during an emergency, all power resides in the executive. If there had been an emergency, the state would not legally have been able to do anything. He told no one where he was, and he was out of communication. If the airplane had crashed or if he had been killed in Argentina it would have taken perhaps weeks for the state to replace him.
So please, Governor Sanford, step down. Although you have made some very wise decisions, the recent adventure you went on shows that you are unfit to govern.
And if you don't, I urge the South Carolina legislature to work towards an impeachment.