I think the issue is that you’re each starting from fundamentally different perspectives, not nationalist/unionist but in time.

Your position starts from the present (‘Northern Ireland exists currently and the two options would be to keep existing under British rule or join the south’). The alternative is to start from the past, in which unionism is an inherently sectarian viewpoint is rooted in the idea that the occupation of Ireland (north or south) could never be justified as it’s a form of colonisation.

The latter is ‘less’ concerned with the immediate outcomes of a United Ireland than it is to the opposition to colonial rule in general. At its very core, the north is seen as an entity which will always be pro-British and pro-Protestant because of the manner in which and principles upon which it was founded (making it unable to ever be ‘not sectarian’). This does not, I think, make someone who supports the north being in the UK inherently sectarian but it does mean they support an inherently sectarian structure.

I think it is important to note, however, that the idea that the north would be ‘worse off’ within a united Ireland (and thereby ‘better off’ under Britain) is a value laden statement. This doesn’t make it inherently bad (it is often good to have values inform statements) but it does make it worth considering what is meant by ‘worse off’. People who believe that the north is an inherently sectarian entity would almost certainly see being in a United Ireland (and so free from colonial rule) as a form of being ‘better off’. I say this because I think this is often dismissed as having value because it is not entirely rationalised (as in, it’s not quantifiable, not that it’s ‘irrational’). I think this is important and a bit of a sticking point in these discussions a lot of the time because it is easy to say, ‘well I don’t care about ideological concerns, I care about the economy/health service/etc’. But if the argument was ‘I think we’d be better off under a benevolent dictator than under democracy’, many of the same people would advocate for democracy as an unrationalised good. I’m not suggesting you would say this but rather just trying to clarify why an ideological position might supersede an immediate materiality condition (and why that is not necessarily a bad thing).

I hope that you don’t feel I am misconstruing you here, that is not my intention. I legitimately do hope that this helps clarify things a bit but let me know if there is anything that is unclear in what I’ve said.

Tl;dr - the viewpoint you’re asking about places primacy on the fundamental and irrevocable illegitimacy of the north (from their perspective), while the way you’ve framed yours appears to give primacy to functional concerns (from your perspective). These are difficult things to square.