Latham's sexually explicit tweet was in response to an earlier post that quoted the independent state MP describing him as a "disgusting human being".
Greenwich claims Latham's post defamed him by claiming he engaged in disgusting sexual acts and was unfit to be a politician.
He also alleges that statements made by Latham and published in the Daily Telegraph in April 2023 further defamed him by accusing him of grooming children in schools.
Appearing on the witness stand Wednesday, Greenwich said Latham's defense made him angry and sad while making his stomach turn.
"I've been in public life for a long time," he told the court.
"I've... tried to make it clear that gay people are normal, we're just like everyone else."
He denied Latham's attempts to say he had gone to schools to talk to children about sexual activity, he said.
"I don't and I wouldn't."
The online clash between the two politicians came after violent protests outside a church in Sydney's south-west, where Latham was giving a pre-election speech in March 2023.
Around 250 counter-protesters, mostly men, violently attacked police and 15 LGBTQI protesters who had set up outside Belfield church, the court heard.
Opening the case, Greenwich lawyer Matt Collins KC said Latham's tweet had unleashed a torrent of abuse, including death threats, despite it only being online for two hours and 20 minutes.
"Our case is that at (that moment), the course of Mr Greenwich's life changed," he told Judge David O'Callaghan.
The former One Nation leader had played on two tired stereotypes - that gays were somehow depraved and a danger to children - in making his comments, Collins said.
The lawyer said that while his client had attempted to engage in genuine debate arising from the violent incident, his opponent took a different route.
"Mr. Latham, instead of participating in that debate, went as low as possible," he said.
The abuse, both online and in messages to his Sydney constituency office, included claims Greenwich was a hairdresser and a pedophile and that death was too good for him, the court was told.
The violent nature of these words caused serious harm to the independent MP, Collins said.
"As a result of Mr Latham's conduct he suffers panic attacks, feels anxious and lacks confidence," the solicitor said.
"He becomes overly emotional, cries, and is reluctant to attend large gatherings out of fear."
Under cross-examination by Latham's lawyer, Keiran Smark SC, Greenwich denied that his initial comment to a Sydney Morning Herald reporter calling Latham a disgusting human being was intended to provoke.
"This was the first case of political violence in an election period in Australia for some time," Greenwich told the court.
"There was a campaign by Mr Latham aimed at vulnerable members of the LGBT community.
"Those two things combined, I think my assessment was appropriate."
The former federal Labor opposition leader will not take the stand.
