The Unionists most relevant here are the Northern Irish ones, not the Scots or English. For most Brits, the idea of Northern Ireland having a special status is essentially baked into the idea of the integrity of UK anyway, and the government would have probably seen some quirky customs exceptions over there in a transition deal as an acceptable price to pay for having a Brexit agreement if they didn't depend on DUP votes to stay in power. But Irish unionists passionately hate the idea of an Irish Sea customs border for the same basic reason Irish republicans hate the idea of a customs border between Northern Ireland and the Republic of Ireland: it's a symbolic split from a nation they consider themselves to be part of and whose identity a lot of blood has been shed over in living memory.

Sure, customs checks at ferry and airports to the UK mainland would entail considerably fewer practical delays, smuggling opportunities and security issues than customs checks between a village and its nearest supermarket on the Irish border, but unionists can legitimately argue an Irish Sea border violates their understanding of the spirit of the Good Friday Agreement. Though it'd also be perfectly fair to ask their politicians who all backed Brexit what did you expect to happen?