Immoral actions are not criminal. Why should Bill Clinton be questioned for acting immorally?
The question with the sex issue is not the affair, it is the perjury and cover-up. Both are reasonable scenarios that are being investigated. Anyway, Congress and the Supreme Court have clarified what applies to impeachment (in the constitution it is listed as “high crimes and misdemeanors” more info.). They have defined impeachment offenses generally as: criminal actions, serious abuses of power, and grave misconduct in office. The presidency too comes with an image. This image, if good, reflects the people who chose this leader. This too applies if the image is bad. 5.