Val DiGiorgio's resignation was announced by the state Republican Party.
In a letter to top party officials, DiGiorgio, 51, said the report by The Philadelphia Inquirer contains "gross mischaracterizations" of what he said were "mutual consensual communications."
"My resignation should be in no way confused as confirmation of these mischaracterizations. I intend to rigorously defend myself against these assertions and protect my family, my colleagues, and the party from this private matter," DiGiorgio wrote. "I extend my deepest apologies to my family and colleagues for this unfortunate distraction."
Irina Goldstein, 35, first received a Facebook friend request from DiGiorgio, who is married with children, in October after she commented on a group photo that included him, the Inquirer reported.
They began exchanging photos and text messages in a conversation that quickly turned flirtatious and sexually charged on both sides.
DiGiorgio repeatedly commented on her looks.
"Well you also happen to be nice to look at," DiGiorgio wrote.
DiGiorgio also turned down her offer to connect through another social media site, writing, "Your too beautiful for me to follow everywhere."
The Inquirer said they stopped communicating in February after she told him his messages had amounted to him "sexually harassing" her. The Inquirer said it reviewed more than 150 pages of messages, provided by Goldstein, including original messages on her cellphone.
At one point, Goldstein mentioned an aggressive sexual act and requested that DiGiorgio send her an explicit photo, the newspaper said.
DiGiorgio sent a photo, which Goldstein said she saved because she "didn't really feel safe with this man" and that their interactions put her "in a weird predicament."
They never had physical contact and she broke off their flirtation soon after he sent the photo, saying, "I prefer to do the fair and right thing."
"I got it," DiGiorgio replied. "Was having the same thought."
Rumors about their interactions began to circulate, prompting a complaint from DiGiorgio in February, the Inquirer reported.
"Anyway, people increasingly are telling me that you, or a friend of yours, have been talking about communications between you and me. So that's a problem too," he messaged her on Facebook. "And the problem is, some of these people talking about this are not my friends."
She continued seeking his help with her campaign, but DiGiorgio said he could not get involved in primaries.
Goldstein, in a Feb. 26 message, responded that his generosity amounted to him "sexually harassing" her and that she had been "afraid of retaliation and rightly so apparently."
Asked by The Inquirer about her claim of retaliation, Goldstein said she had felt harassed by calls from people asking who she had told about her communication with DiGiorgio.
In a reply to her, DiGiorgio denied any improper behavior.
Joel Frank, general counsel for the state Republican Party and a lawyer for DiGiorgio, called Goldstein's claims "a mischaracterization, incomplete and defamatory" in a letter last week.
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