I agree that Bill Clinton should not have been forced to testify over Monica Lewinski by itself, but that was not what got him in trouble. He had been sued by Paula Jones for sexual harassment and had been ordered to testify at her civil trial where other incidents of his infidelity were brought up as evidence. It was there that the original crime took place. He lied deliberately in a civil trial that had been brought by a private citizen against him.
He admitted to lying in court. The judge in the case referred him to the Arkansas State Bar, which disbarred him for five years, if memory serves. He was also permanently banned by the Supreme Court so that he could no longer bring cases before them. He was clearly guilty of everything he was accused of at trial, and eventually confessed to perjury.
The Clinton machine did everything it could to destroy Paula Jones, calling her trailer trash, destroying her reputation, etc. in order to win the trial. He basically used the power of the presidency and the powers of his supporters against an ordinary civilian whom he had done wrong.
That is what outraged people. Most people, including myself, couldn't care less about him having affairs in the White House. It may be rather "ew" but most didn't really care. As a defense in his impeachment trial, he and his surrogates successfully turned a rather bland, emotionless prosecutor into a "sex fiend" and made the whole trial about sex. That way, he got the sympathy of the American people. When it's the sex police going after him, of course it sounded ludicrous. But when put in its proper perspective that he broke the law, using the enormous power of his office to wrong an American citizen, that's quite a different matter. It's a case of the powerful using all their considerable leverage against the powerless, something I would think would outrage those on the left.
As Carol said, it's a matter of opinion whether it rose to the level of a high crime or misdemeanor (doesn't actually mean what it sounds like as the Federalist papers indicated the phrase referred to malfeasance or immorality in office, a very low bar).
I personally think it was a mistake to impeach him but he did deserve being disbarred and being forced to pay reparations to the woman he harassed and then tried to destroy.
