What each of us does is the evidence by which others assess us.
Politics covers ethical and practical components of human life.
Politicians represent us in making decisions on these matters.
Many voters therefore judge the ability of a politician to represent their views, based on what the politician does in their life.
This is different from the judgement made on say, someone who works on a factory production line. The way they conduct their personal life is probably not much use in assessing their ability to do the task (though it might have a bearing on their wider capability as an employee).
If a politician has an extra-marital affair, then people have evidence to assess that the politician has the capacity to deceive, not to fulfill commitments, and to be side-tracked.
The insight an extra-marital affair offers into these traits is not significant nor conclusive. We simply learn that the politician is as flawed as the rest of us.
I’ve previously blogged that “normal” sexual transgressions, and normal sexual interest itself, is almost never a catalyst for the end of a political career. Because the insight into the person means they don’t differ markedly from the people they represent.
The further the type of sexual interest, and intensity of its pursuit, varies from ‘normal’, the more likely voters are to assess that the politician varies from their expectation of the sort of person who best represents their interests.
So the sexual activity of politicians is a perfectly legitimate voter interest, but the scale of its disclosure, if at all, depends entirely on the scale of its difference from societal norms and professed values.