The only reason for that is NI never voted/fought for secession from the UK - just relatively small groups without popular support in NI, but with the support of Ireland.
Actually, they did, back when Ireland as a whole voted for independence. That that particular part of the country may have chosen to remain does not, under democratic rules, allow for it to be separated from the main country, much like Scotland and Wales could not have argue to remain in the EU because their respective populations were overwhelmingly in favour of remain.
And to be quite frank, you are probably thinking of the Provisional IRA when you put legitimacy in quotes, where as this was the original IRA, who basically have nothing in common with the Provos (as they were called when I were a lad) except the name.
When Ireland voted for independence, it was officially part of the UK. The UK as a whole would have voted against independence for Ireland. Does that mean under democratic rules Ireland was not allowed for it to be separated from the main country?
We'd be better off asking why unionist don't want to join rather than telling them they have no choice.
I don't think anyone is telling them that: in fact their concerns are absurdly well catered for, though you wouldn't know it listening to the constant complaints.
We can split hairs about what is and is not democratic until the end of time, but the debate takes place in a historical vacuum: the creation of Northern Ireland was a response to a very specific set of circumstances, and I don't think yak-shaving about democratic niceties casts any light.
It is moot anyway, because there is no prospect of changing Northern Ireland's status without democratic sanction. Which vote of course takes place at the pleasure of His Majesty's Secretary of State for Northern Ireland.
It is not quite at the pleasure of His Majesty's Secretary of State for Northern Ireland. It is defined in law that if there are grounds to believe it would pass they would be required to hold a border poll. This could be decided in court, as in if say opinion polling was at 70% and nationalist parts were winning 70% of the votes, and the Secretary of State refused to hold the poll. They could be brought to court and a judge could rule the secretary has no grounds to refuse.
The levels of support for the small groups that fought primarily against small groups on the other side were never really gauged. However the political successors to the only one of those groups that really counted is currently the largest party in Northern Ireland.
The Brits left split polities in quite a few places as they decolonized. Almost as if it was policy.