That whole argument is BS for a few different reasons. First of all, not everything is “democratically legalized”, that’s not the way it works even in a democratic government. The United States didn’t put interracial marriage up for a vote, did they? And second…it actually was not “against the will of the Northern Ireland people” anyway!
A September 2014 LucidTalk poll for the Belfast Telegraph showed that 40.1% of the population supported same-sex marriage, while 39.4% opposed and 20.5% either had or stated no opinion. Of those that gave an opinion, 50.5% supported and 49.5% opposed same-sex marriage.[83] A poll in May 2015 found that 68% of the population supported same-sex marriage, with support rising to 75% in Belfast.[84] A "mass rally", organised by the Irish Congress of Trade Unions, Amnesty International, and the Rainbow Project took place in Belfast on 13 June 2015,[85] with a 20,000 person turnout.[86] A June 2016 poll placed support for same-sex marriage at 70%, while those opposing it at 22%.[87]
A December 2016 LucidTalk poll found that 65.22% of people surveyed supported the legalisation of same-sex marriage in Northern Ireland.[88] However, a majority of Unionist respondents was opposed to same-sex marriage in Northern Ireland, with only 37.04% in favour (with support rising to 71% for Unionists aged between 18 and 24 years of age).[88] By contrast, 92.92% of Nationalist/Republican respondents and 95.75% of Alliance/Green/PBP voters were in favour.[88]
An April 2018 Sky Data poll placed support for same-sex marriage in Northern Ireland at 76%, with 18% opposed.[89]
A 2019 poll conducted by YouGov revealed that 70% of British people agreed that same-sex marriage should be legalised in Northern Ireland (up from 65% in 2018), including 55% of those living in Northern Ireland. Among Conservative voters, 62% (up from 54% in 2018) expressed support for the introduction of same-sex marriage in Northern Ireland. 81% of "Remain" voters said they supported same-sex marriage, compared to 60% of "Leave" voters (this was at a time when in the aftermath of the 2016 EU referendum the issue of Brexit dominated British politics). In Northern Ireland itself, 72% of women supported same-sex marriage, compared to 40% of men.[90][91][92]