I think the SNP have been happy to trade on a quite deliberately-fostered confusion between violent and bigoted Loyalism, Ulster Unionism (let's not forget - and I'm writing as a Scots Catholic of Irish extraction - that non-violent Ulster Unionism is a perfectly legitimate political viewpoint worthy of honest consideration, notwithstanding its long-standing reluctance to give ground when Catholics demanded equal rights under the law), and Scottish anti-secessionism, which they have been happy to conflate for political reasons. Hence the word 'Unionist' being applied quite wrongly in Scotland when in fact Scottish anti-secessionism has a quite different rationale than tribally-based Unionism/Loyalism in Ulster.
Of course the Bucky-drinking, knuckle-dragging Ibrox contingent have made this easier than it should have been, while the more idiotic 'Unionists' among English Tories only make things worse by assuming that anti-secessionist Scots like me are happy with Union flags everywhere, completly oblivious to the toxicity of this in some quarters, particularly in the West of Scotland. Hence now, in the unthiking see-saw way of these things, many cradle Catholics have signed up for the SNP purely to set themselves against the sort of people pictured rioting earlier in the thread. This is very toxic to genuine, constructive political discourse (far from in the interests of Scots Catholics more generally) and has added a very unwelcome injection of Ulster tribalism into Scottish politics - something, as I say, that the SNP have been happy to exploit and that the Tories have failed completely to understand.
For me the arguments against secession are mostly economic but also identitarian - I reject the SNP's attempts to divide the peoples of the UK, pretending moral superiority or difference in values which don't exist (per the annual Social Attitudes Survey). I see no reason why my full identity as a Scot (or sixth generation Scots-Irish, if you must - the Irish famine was a long time ago) can't co-exist quite happily with a British identity dating back to the union of the crowns in 1603 when England and Scotland came under a common monarch, and 1703 when this arrangement was formalised politically. Since my recent retirement (I was 20 years a daily newspaper journalist and 20 years in management consultancy) I'm actually working on a PhD in 17th century Scottish music so I have some familiarity with turbulent 17th century history across Scotland, England and Ireland. I really value the diversity of the UK's constituent nations, but I also see myself as European, a product of western Christendom with an orientation towards our common Roman/Christian heritage (I do read Latin and speak French and German) - unlike some I don't feel I need an EU passport for that, my European connections being far more deep and ancient than anything Brussels can offer.
The economic arguments of course I have outlined above. I don't want my aged mother or my siblings' pensions to be put at risk through SNP economic dishonesty, nor do I want Scotland to turn into a failed state, which is where it would seem to be heading anyway thanks to the SNP's serial incompetence and general disinterest in actually running the country as well as it could be run, as opposed to bribing indyref voters with freebies from a Barnett dividend that would disappear on independence day.