Shirtless GOP lawmaker caught trolling Craigslist to cheat on his wife.

Classy.

Upstate Rep. Christopher Lee abruptly quit his job yesterday after he was exposed trolling for an extramarital tryst on Craigslist -- using his real name and Gmail account.

In a mind-boggling blunder, the Buffalo-area Republican, who is married with a young son, responded to an ad in Craigslist's "women seeking men" section in DC. He claimed to be a "divorced" "lobbyist" and a "fit fun classy guy."

As proof, Lee sent along a cheesy photo of himself, flexing his muscles and posing shirtless in front of a mirror.

Lee's cover was blown when his would-be conquest did an Internet search, discovered he was lying, and sent the treasure trove of their correspondence to Gawker.com, which yesterday posted it online. Lee's staffers -- who initially suggested the lawmaker's e-mail had been hacked -- dodged media queries before announcing the resignation late yesterday.

"I regret the harm that my actions have caused my family, my staff and my constituents," Lee said in the statement, which did not address the Craigslist brouhaha. He later told Fox News Channel, "I gotta work this out with my wife."

Lee's downfall apparently was sparked by his exchange with a 34-year-old single woman of "black/Irish" descent, who had advertised online for a "financially & emotionally secure" man.

"Will Someone Prove To Me Not All CL [Craigslist] Men Look Like Toads," she wrote in a Jan. 14 ad.

"Hi, Hope I'm not a toad. :)," Lee responded.

"I'm a very fit fun classy guy. Live in Cap Hill area. 6ft 190 lbs blond blue. 39. Lobbyist. I promise not to disappoint."

Lee, who represented Amherst, NY, is 46 -- although the woman he'd apparently hoped to romance had specifically said she wanted a man between 30 and 40.

At some point, Lee sent the photo of himself, posing in dress pants with a bare chest.

"I just took this one . . . I'm relaxing at home," Lee e-mailed.

"So do you always send shirtless pics to women from cl?" the woman responded. "Sorry. It's all I had," Lee replied.

The Gmail address he used matched the one posted on Lee's Facebook account.

When the woman asked if he was divorced, Lee answered, "Yes," and said he had a child.

In reality, Lee is married. He, his wife, Michele, and their young son recently moved into a new, $650,000, five-bedroom, four-bath home in the Buffalo suburb's Pomeroy Park area, public records show. According to his e-mail exchanges with his new Craigslist friend, Lee wasn't a first-time philanderer.

When the woman -- reportedly a college grad and government employee -- asked Lee when his last date was, he replied, "Lol. last Monday. She was not as advertised. Lol . . . how do people think you aren't going to figure it out once you see them in person."

In June 2009 -- shortly after he helped pass the Student Internet Safety Act -- Lee wrote an op-ed piece warning kids about the dangers of the Internet.

"Responding to what may seem like a friendly e-mail . . . can have serious consequences," he wrote. "Private information and images can so easily be transmitted to friends and strangers alike."