I would need a lawyer's view on it (maybe a few), but from what I remember it was that he lied under oath that got the ball rolling.
One of the persistent lines is that
Clinton's impeachment was about sex.
Article I charged that Clinton lied to the grand jury concerning:
the nature and details of his relationship with Lewinsky
prior false statements he made in the Jones deposition
prior false statements he allowed his lawyer to make characterizing Lewinsky's affidavit
his attempts to tamper with witnesses.
Article III charged Clinton with attempting to obstruct justice in the Jones case by:
encouraging Lewinsky to file a false affidavit
encouraging Lewinsky to give false testimony if and when she was called to testify
concealing gifts he had given to Lewinsky that had been subpoenaed
attempting to secure a job for Lewinsky to influence her testimony
permitting his lawyer to make false statements characterizing Lewinsky's affidavit
attempting to tamper with the possible testimony of his secretary Betty Curie
making false and misleading statements to potential grand jury witnesses.
I still believe that if he had said, "It's none of your business, but yeah, I did it. In fact, i'ma go to the office and do it again," my father-in-law (lifelong Democrat) would have said, "Yeah. Old guy, gettin' some on the side with a young woman! That's why I like that guy."