Politicians make laws governing society, determining what behaviours are right and wrong. They frequently prohibit or punish people from doing things that the politician considers wrong but an individual may have no problem with (fees on plastic bags, pesticide bans, zoning regulations, squeege kids). They also may decide to allow or not act on activity that others find is wrong or harmful (gay marriage, approving bad development, tolerating bad TTC customer service).
When a politician is caught doing something wrong, whether illegal or not, people feel that they lose some (maybe just a tiny bit) of their legitimacy in governing the behaviour of others. It doesn't have to be something illegal like beating your wife, it could be cheating on someone, telling a citizen to f-- off, lying, making racist statements about Africans etc. Although no person is perfect, we have higher standards for our leaders (not just politicians, but teachers, ministers etc) and are surprised when we find out that they are human just like us, especially when they cultivate a clean image. We may choose to care or not care, depending on the context, how likeable they are, on our own value system and most especially our political leanings.
I doubt that many people would vote or not vote for someone because they cheated on their girlfriend, but combined with the lying, using the girlfriend as a political prop, age difference etc, it hurts their image, raises questions about their character, and at the very least brings back down to earth anyone who thought he was the perfect candidate who could do know wrong.
