Northern Ireland's ban on same-sex marriage breaches human rights legislation and marginalises gay people as "biological oddities", a court has been told.
Two couples, Grainne Close and Shannon Sickles and Chris and Henry Flanagan-Kanem, are seeking to overturn the region's prohibition on gay marriage.
They were, respectively, the first and second couples to enter into a civil partnership after Northern Ireland became the first part of the UK to make that option available in December 2005.
Following the Yes vote in May's referendum on marriage equality in the Irish Republic, Northern Ireland is the only part of the UK or Ireland where civil marriage is denied to same-sex couples.
Opening the application for judicial review, barrister David McMillen QC said: "Northern Ireland stands as a blot on the map; a backward-looking, divisive and divided society."
The prominent case is being heard at Belfast High Court by Mr Justice O'Hara.
The devolved Stormont Assembly has repeatedly refused to legislate on the contentious issue.
Although the majority of MLAs voted in favour of introducing gay marriage when it was debated for a fifth time last month, the proposal fell because unionists who oppose the move deployed a controversial voting mechanism to effectively veto it.
The petition of concern, which allows 30 Assembly members to block a proposal is "manifestly inappropriate in this particular area," argued Mr McMillen, who is representing both couples.
Proceedings are being taken against the Department of Finance and Personnel on grounds that the ban contravenes entitlements to marriage and a family life under the European Convention on Human Rights.
It amounts to State discrimination of an already marginalised section of society, the court was told.
"The love and life-long commitment of these four applicants is the same as any other couple," Mr McMillen said.
"It is demeaning and offensive that their unions have been relegated to a second-class status - civil partnership."
Both couples have been together since 2002 and the male applicants, who were not in court, also have a young son, it was revealed.
Mr McMillen said: "If history, and in particular the history of Northern Ireland, tells us anything it is that discrimination of a being and private life is wrong.
"It keeps individuals apart from mainstream society and is corrosive to the very fabric of society."
It was also claimed that the absence of marriage equality had detrimental consequences on the health of gay couples and labelled them "less worthy" than heterosexuals.
"Like biological oddities who do not fit into society," the lawyer added.
Earlier this year about 20,000 campaigners marched through Belfast city centre demanding a change in the law.
The courts have a duty to intervene because the Assembly is unlikely to grasp the issue, the judge was told.
The landmark legal action is being heard in tandem with another case in which two men who wed in England are seeking to have their marriage recognised in Northern Ireland.
Mr Justice O'Hara is expected to rule on both cases next year.
The judge said: "There are significant differences between the cases."
Ms Close and Ms Sickles sat in the public gallery alongside gay rights campaigners including comedian Nuala McKeever.
Amnesty International, which is supporting all four applicants, has described the case as "hugely significant".
Patrick Corrigan, Amnesty's Northern Ireland programme director, who was in court, said: "Following the repeated failure of the Northern Ireland Assembly to legislate for marriage equality, couples have been forced into the courtroom to demand equal treatment before the law.
"It is unacceptable that they have been obliged to sue the Government in order to have what the rest of society takes for granted - for the State to recognise their right to get married.
"With politicians having abdicated their responsibility to deliver equal treatment for same-sex couples, it is now over to the courts."
Gavin Boyd, from the Rainbow Project, an LGBT support group, said the legal action was a last bid to achieve marriage equality.
Meanwhile, the judge also heard that both sides had agreed to limit any costs.
The case has been adjourned until Friday when a lawyer for the DFP will outline their case.