How about his lying under oath?
If I recall, that happened as a result of his giving testimony related to the impeachment itself, which never should have happened in the first place. There are parallels to the persecution of General Flynn for a "process crime" instead of an actual crime. Special Prosecutor Starr's team started out with Clinton's sex life and beat the thing like a dead horse until they finally elicited some legal-sounding tbing to hang the whole political persecution upon.
Make no mistake, I think he was and is a world class scumbag. But the tool of impeachment should properly be limited to acts that actually imperial our Constitution or national defense, which his sex life did not. Dragging all the testimony about oral sex into the public and printed in the daily newspapers had nothing to do with improving the lives of citizens, but actually triggered a drastic spike in oral sex and STDs by teenagers, and did lasting damage by unbalancing relations between the Legislative Branch and the Presidency.