Just out of interest TTT, how much would your tune change if May, or whoever, came back from the negotiating table with the offer of being, effectively, out of the EU in name only? Still paying continued contributions of a few billion a year, still accepting free movement, still deferring to the EU Courts on various matters, still accepting a reasonable proportion of EU law? Would you be like "oh well, we're out of the EU officially so that's all right" -- or would you want to have a meaningful say on what you think of that deal?
I rather suspect that it's the latter. In practice I doubt that the deal on offer will be remotely so "bad", from your perspective, but in theory it *could* be something resembling that. And in that case, for the government to argue that the referendum gave it a mandate for such a deal is so outrageous that of *course* you'd want another vote to tell them to *** off.
The same is true the other way round -- perhaps less urgently, but the principle still stands. A second referendum isn't mandatory, but it shouldn't be dismissed out of hand because of some hubris that you are going to get what you want.