The point of a Presidential candidate's infidelity is that such behavior shows a lack of self-control and a penchant for stupid, dangerous risks.
Sure, the guy's under pressure. Sure, he loves to be adored. But as our nation's leader, he is supposed to put the greater good first. That's what is so problematic about Senator Edward's infidelity. He had an affair during a time when it was most likely to be discovered and would hurt his career and family the most. And he did it anyway.
I'm sure that as President, Senator Edwards would also have heard the siren song of invincibility. Would that be the time to open himself up to blackmail because of an ill-advised dalliance? I hope not. The President of the United States represents us and negotiates on our behalf. This country needs someone they can trust to do that.