So what’s undemocratic about having a second referendum?

One of the things that really baffles me about British politics at the moment is Brexiteers’ insistence that putting the Theresa May deal to a second referendum would be “undemocratic”. Au contraire, I think it’s the only democratic thing to do. One of the basic principles of good interface design in software is that whenever a user indicates that s/he deliberately or inadvertently intends to do something that would be damaging or that s/he might regret, the system first puts up a dialog box to check that the signalled action was in fact deliberate and that the user is comfortable with the consequences. If there were a second referendum and a majority voted to leave on the May terms or remain in the EU then that would be a truly democratic decision (you could call it informed consent) and even those of us who disagreed with it would have to accept it.