It is the only part of the UK in which what campaigners term marriage equality has not been legalised due to religious-inspired opposition from its largest party, DUP.
Justin McAleese was a prominent campaigner in the Irish Republic's watershed referendum campaign in 2015, when he discussed his experience growing up as a gay man.
Mr McAleese said he had a "huge problem" with the level of leadership in the Catholic Church but urged advocates of change to engage in difficult conversations with those who think differently.
He said: "Faith has no place in secular, registry office marriage."
The campaigner attended an event in Belfast as part of the annual Pride festival.
The Democratic Unionists, Prime Minister Theresa May's close allies, hold that Christian marriage is between a man and a woman and have used a parliamentary mechanism at Stormont to veto efforts to legislate in Northern Ireland.
Mr McAleese said the Church of Ireland and Catholic Church was entitled to its definition of marriage but should not be able to force that view on everyone else in the secular world.
He said: "Coming at this debate through the prism of one definition of marriage from the tradition of religion is simply not good enough when you are talking about civil registry office marriage in secular Northern Ireland."
He added: "We need to talk about marriage... one kind of marriage, everyone equal, that would be progress."
When the now-suspended Stormont Assembly was operating its members voted in favour of change, while public opinion polls show a majority of above 70% back social liberalisation.
Former president Mary McAleese was an outspoken advocate for same-sex marriage during Ireland's referendum campaign which saw such unions legalised.
Her son Justin has said he wanted to win a seat in the next Irish general election.
He called for a proper debate around sexual health and education in the Republic, claiming the authorities were "away behind" on addressing some of these issues.
"You should leave school knowing how you can ensure your own sexual health.
"There is a lot of work to be done but nothing has happened since the referendum."
Earlier, Barnardo's became the first children's charity in Northern Ireland to publicly voice support for the recognition of same-sex marriage.
With devolved government at Stormont remaining in suspension since January 2017, earlier this year Labour MP Conor McGinn tried unsuccessfully to force the policy through Westminster via a private members bill.
Koulla Yiasouma, Northern Ireland Commissioner for Children and Young People, said: "It is clear the current situation in terms of same sex marriage in Northern Ireland is untenable and legislation needs to be enacted as soon as possible to address this discrimination and its direct and indirect effect on children and young people."
