They might start to do this now, however, since there are some figures available that strongly confirm this belief. Popular international dating community Zoosk conducted a massive survey with almost 13000 of its members, quizzing them on their attitudes to relationship morality.
In line with the stereotype the French respondents were the least concerned with their leaders' sex lives, with 4 out of 5 singles saying they wouldn't care if a politician were unfaithful. Those in America were far more judgmental, however.
The survey turned up lots of other interesting insights into various national attitudes to infidelity. Some of these confirm old stereotypes, while others contradict them.