Daniel Hooper, who, the paper reported, performed a "solo sex act" on Skype with a glamour model while he was dating Coronation Street actress Kym Marsh, is suing the paper for invading his privacy, according to Press Gazette.
The newspaper, which has campaigned with its sister paper The Sun for the Act to be abolished, is using the legislation to defend itself, despite its own columnist calling the Act "twisted" and "an affront to human decency".
The newspaper published a story in May 2014 saying it had seen a 10-minute video showing Hooper, a personal trainer, masterbating while talking to the unnamed model in a web chat. She had recorded the encounter and was trying to sell the video online.
Articles from other newspapers reporting the news are visible online, but The Sun's piece does not appear to be on its website.
The glamour model claimed that Hooper was romantically involved with Marsh when they had their encounter, but Hooper said he had not yet begun dating the actress.
According to Press Gazette, which first reported the news, Hooper's claim is that The Sun on Sunday showed "flagrant disregard" for his private life and was presented it a "sensational and especially intrusive manner".
He alleges that the story caused him to suffer "substantial distress and embarrassment", Press Gazette reported.
But The Sun on Sunday reportedly argues that Hooper had no right to expect that the web sex would stay private when it was obvious she could have recorded and shared it.
The paper is citing its right to freedom of expression to back its decision to publish the article describing Hooper's encounter with the unnamed model.
The newspaper claims that the publication was justified under its right to freedom of expression, which is protected under article 10 of the European Convention of Human Rights.
The Sun on Sunday's right to express itself is greater than Hooper's right to privacy, the paper's publisher News Group Newspapers reportedly claims.
Its defence also reportedly notes that article story did not contain any pictures or video from the incident.
The Sun and Sun on Sunday have this year voiced their support for David Cameron's promise that the Tory government will ditch the act, described by The Sun as "hated".
In early 2015 a columnist in The Sun on Sunday called the Act "twisted" and "an affront to human decency".
Tony Parsons wrote that "contradicts everything the British believe about fairness and justice" and gives "comfort and aid to the perpetrators of evil and insults the good, the innocent and all the victims of crime."
The paper has stated that the Act unfairly protects the rights of foreign criminals, rather than victims of crime.
The Huffington Post UK contacted The Sun on Sunday's parent company News UK, which was unable to comment ahead of publication.
The Conservative general election manifesto promised to abolish the Act, which makes the European Convention on Human Rights enforceable in UK courts, and replace it with a "Bill of Rights".
In the Queen's Speech, the monarch said the government would "bring forward" the plans, rather than commit to removing the act, in a move seen as a backtracking attempt after vocal objection to the idea.
The move came after campaigners, celebrities and MPs criticised it, and more than 240,000 people signed a Change.org petition opposing its abolition.
After a long silence on the matter, the government still wants to abolish the act when Dominic Raab, Parliamentary Under-Secretary of State at the Ministry of Justice, confirmed this month that plans are continuing.