The Clintons threw everything at Newt including, yes, dirt. But it was more than just dirt. The Clintons demonized conservatism and for senior citizens, soccer moms and others more gullible, it worked.
Yes, demonization played a huge factor in public perception of Newt, but the adultery dirt was true, and the Clintons used it to blackmail Newt when he came to negotiate with the White House about signing certain legislation as written.
I doubt the demonization had nothing the do with his resignation. I think he realized he had lost all effectiveness as Speaker because of the blackmail, and he resigned so the House could find a more effective Speaker.