Unless your name is William Jefferson Blythe Clinton. Then it's OK and "no one's business". Most corporations have rules against that sort of thing as well. It's bad for morale, of everyone else that is. The military has similar regulations. To the point that if a couple is "involved" and one becomes an officer, there is a time limit for them to either get married, or break it off. Most chose to get married.
That’s what feminists were screaming about when it was Clarence Thomas. That when there’s an imbalance of power, it’s a crime. This rule was, like you said, suspended when Clinton did it. Then it was OK.
What clinton did with monica was not illegal. Lying to a grand jury about it WAS illegal.