A judge was told Health Minister Edwin Poots' stance is irrational and discriminatory.
Lawyers for an unidentified gay man are seeking a ruling which would bring Northern Ireland's policy into line with the rest of the UK.
They also argued that the risk of spreading infection through blood was greater among heterosexual groups.
The complete prohibition, put in place during the 1980s AIDS threat, was lifted in England, Scotland and Wales in November 2011. It was replaced by new rules which permit donations from men whose last sexual contact with another man was more than a year ago.
The 12-month deferral was left in place following a Government Advisory Committee report.
It identified a much shorter a window period during which infection with blood-borne viruses could not be detected.
Mr Poots has so far maintained the ban in Northern Ireland, declaring it was to ensure public safety. Attorney General John Larkin QC, representing the Department and the Minister, is expected to argue that no definitive decision has been taken.
But a lawyer for the gay man who has brought judicial review proceedings claimed Mr Poots did initially make a "knee-jerk" decision despite advice from his officials to join England, Scotland and Wales in accepting recommendations for a lifting of the ban.
David Scoffield QC said there was pressure for a joint, UK-wide ministerial response adopting a single position.
"We say that the maintenance of the ban represents unlawful discrimination against homosexual men, principally on the basis of their sexual orientation as compared to heterosexual men," he contended.
The barrister, whose client was granted anonymity due to his perceived vulnerability, rejected a claim that the prohibition was centred on sexual behaviour rather than sexual orientation.
He continued: "In the final analysis the different treatment of homosexual men in this case is simply not justified by the additional risk or, we would say, the claimed additional risk the removal of the lifetime ban would cause."
Mr Justice Treacy was told the lifetime prohibition was disproportionate, with an estimated one additional HIV infection per billion blood donations if a 12-month deferral period was introduced instead.
"There's an extremely slight, indeed a minuscule increase in risk," Mr Scoffield said.
He further argued the evidence even suggests a deferral period could lead to greater compliance and a reduced risk.
Although any man who has sex with another man cannot ever give blood under the current rules, even if they have been tested for infection and have been in a monogamous relationship, the court heard. But those who have sex with either a prostitute or an intravenous drug user are only put off for 12 months.
"There's an emerging theme: if you have what might be called risky sex in terms of infections you are deferred for one year," Mr Scoffield said.
He added that the risk of HIV being spread through blood transfusion was less in Northern Ireland than in England, Scotland and Wales.
"We say there is no proper basis for the Minister concluding that he will not follow the approach of the other UK authorities when the risk here is proportionately much less.
"The approach of the Minister has adopted is properly to be regarded as unreasonable."
The case continues.