Well if you're going to be in public office, you're going to have to set some kind of example, some kind of higher standard. It comes with the territory.
It's not some universal rule written in the stars. For a lot of continental Europe someones private life (yes, even a politician's) is exactly that... private. Unless it directly interferes with their day job it is no-one's business but theirs.
To use a controversial example: Martin Luther King, Jr was a plagiarist during his academic career, his subsequent publications and even the "I have a dream" speech borrows liberally from one of his friends and had multiple extramarital affairs. In today's political world there's a good chance he would have never got off the starting blocks because of that.
As I understand it Europe has it completely 180º. The French PM's mistress often appears in public with him from what I heard.
That may be a case of the meaning getting lost in translation; The French Prime Minister's wife is semi-officially known as the "mistress of Matignon". As far as I know Fihlon (the French PM) has never been accused of being unfaithful.
Nicolas Sarkozy (the French President) has been accused of adultery before in 2005/6, but at the time he and his wife were separated after she had already had an affair. As he's currently married to Carla Bruni
and the couple are on somewhat of a charm offensive I doubt his eye is wandering.
The real star of the "Private = Private" point of view is the Italian Prime Minister Silvio Berlusconi
. It would take too long to go through all the issues he's faced but a quick list includes:
1) Links to the mafia
2) Choosing his EU elections candidates primarily on looks: several were (ex)models with no political experience
3) Several affairs, including the most recent being with an 18 year old model
4) "Entertaining" prostitutes
5) Having near total control of the Italian media
6) Doing the cuckold symbol behind other politicians back
7) Saying German politicians would fit perfectly as guards in a concentration camp
8) Saying companies should invest in Italy as they have the most beautiful secretaries.
9) Accusing the Chinese of eating their children... and then as a retraction saying instead they boiled them to fertilise their fields.
10) Complimenting Obama on his "sun tan"
11) Playing hide and seek when receiving politicians
12) Saying that people who lost their homes in a recent earthquake should treat their refugee camp as a "camping holiday".
And yet he is one of Italy's most popular ever politicians...