Same-sex couples denied the opportunity to marry in Northern Ireland are being subjected to unlawful discrimination, the Court of Appeal was told.
Senior judges were told a failure to introduce rights available to those living in the rest of the United Kingdom cannot be justified.
The claims were made as two gay couples renewed their legal challenge to the ban on them getting wed.
Grainne Close and her partner Shannon Sickles, along with Chris and Henry Flanagan-Kane, are seeking to overturn a ruling that the prohibition does not breach their human rights under European law.
In 2005 they became the first couples in the UK to enter civil partnerships, cementing their relationships in ceremonies at Belfast City Hall.
But unlike England, Scotland, Wales and the Republic of Ireland, Northern Ireland has still not legalised same-sex marriage.
Last year a judge dismissed the case after finding that it was a matter for the Stormont administration rather than the courts.
Although the challenge is being taken against the Department of Finance, counsel for the two couples insisted there was a "pan-UK basis" in the continued absence of a functioning Executive.
Ronan Lavery QC argued that the failure by the state to include the people of Northern Ireland in same-sex marriage legislation breached their human rights.
"It's unlawful discrimination against the applicants on the basis of their sexuality," he contended.
Prior to the collapse of devolution MLAs held a number of votes on the issue - with a narrow majority in favour of the move back in November 2015.
However, the Democratic Unionist Party deployed a petition of concern mechanism to block the motion.
The court heard that 15 out of close to 50 jurisdictions within the Council of Europe have now brought in same-sex marriage.
But Strasbourg has left it up to individual states to decide on the policy.
Pressed to define the obligation allegedly being breached, Mr Lavery submitted that once the marriage rights were brought in they should apply equally to all parts of the UK.
He rejected "this notion that gays and lesbians should be happy enough with civil partnerships".
The challenge is being resisted by lawyers for the Department and the Attorney General, John Larkin QC.
Mr Larkin argued that the applicants lacked legal standing to bring the action.
"What we are dealing with is not an act of the Assembly," he told the three judges.
As the appeal continued, the two couples told of their continued dismay at the bar on getting married.
Speaking outside court, Ms Sickles said: "It's exhausting to feel discriminated against, its frustrating, it's enraging and its unfair."
Her partner, Ms Close, added that they were left relying on the courts to "step up to the mark" in the absence of Stormont.
"We don't want to be here, but it's our only avenue," she said.
Tony McGleenan QC, for the Department, accepted differential treatment between heterosexual and same-sex couples.
He told the court the legitimate aim was about preserving so-called traditional marriage.
Provisions are made for same-sex couples through the availability of civil partnerships, the barrister argued.
Mr McGleenan is set to make further submissions when the case resumes on Thursday.
Outside court solicitor Paul Pierce of KRW Law, representing the two couples, said it was unfortunate they had to bring a case to try to obtain equality with others.
"Throughout these islands people in same-sex relationships are free to marry - except in this jurisdiction," he added.
"It's regrettable that society is unable to grasp the nettle of change and allow same-sex couples to enjoy the same rights as those in heterosexual relationships.
"As the Lord Chief Justice stated when questioning counsel for the Department about justification for the alleged discrimination, if the state was to abolish marriage generally there would be riots in the streets."