Assemblyman Mike Duvall said Wednesday in a statement that his comments were inappropriate and have become "a major distraction" from the work lawmakers are trying to finish during the final week of their session.
Duvall says it would not be fair to his family, constituents or friends to remain in office.
"I am deeply saddened that my inappropriate comments have become a major distraction for my colleagues in the Assembly, who are working hard on the very serious problems facing our state," he said. "Therefore, I have decided to resign my office, effective immediately, so that the Assembly can get back to work."
Duvall, whose is married with two adult children, made the comments to another lawmaker on July 8 during a break in a committee hearing that was caught on a live microphone.
In a recording broadcast by KCAL-TV in Los Angeles, the 54-year-old lawmaker talks about a recent sexual escapade with a woman he says is 18 years younger than him.
"I like spanking her," he is heard to say on the videotape. "She goes, 'I know you like spanking me.' I said, 'Yeah, that's cause you're such a bad girl."'
He then brags about an affair he is having with another lobbyist.
Duvall also discusses the age difference between himself and the lobbyist, who is not named on the videotape, and says he joked with her that she was getting old after turning 36. Duvall said he told her, "I am going to have to trade you in," reports CBS station KPIX-TV in San Francisco.
The lobbyist in question is rumored to work for Sempra Energy, a San Diego-based energy company. In an email, Sempra said it was investigating the claims.
"The employee has denied the speculative media reports. Our investigation will be conducted to ensure not only that our policies on employee conduct are strictly adhered to, but also that our employee is treated fairly," the company said.
Duvall was vice chairman of the utilities committee, which had been considering one of the more hotly contested bills in the current legislative session. The bill, which is still being debated, would require utilities to generate 33 percent of their power from renewable sources by 2020. Many utilities oppose it, reports KPIX-TV.
On July 6, Duvall voted against the legislation.
Duvall received a 100 percent rating from a conservative advocacy group, for his votes on legislation considered pro-family during the 2007-08 legislative session.
The group's executive director, Karen England, said the scorecard is based strictly on lawmakers' voting records, not their personal conduct, and that Duvall consistently voted in favor of legislation the group considered pro-family, according to KPIX-TV.
"It's unfortunate that he wasn't living it out," she said.
Duvall won an ethics award from Chapman University in 2000 for "demonstration of the highest standards of ethical integrity."
Assembly Speaker Karen Bass has asked the Assembly Ethics Committee to investigate the incident.
"I did speak with him and he was extremely remorseful and regretful," Bass said at a press conference. "We have such big issues before the Legislature and to have this become a distraction, he felt his responsibility was to step aside."
Assembly Minority Leader Sam Blakeslee asked Duvall to step down as a member of the Republican caucus leadership before his resignation on Wednesday.
On Thursday morning, all information on the former Assemblyman, including his photo, biography and press releases had been removed from the State Assembly's Web Site.