One reason1 for all the agonising is that having the “yes” answer allows a more positive campaign, as we’ve seen recently (even if it doesn’t always lead to victory).
So the stay-ers get the yes option:
But why should it restricted to a boring yes/no response, when there’s so many more possibilities? Here’s an option that would suit computer nerds:
Or there’s always:
I wonder which one they will choose? Maybe we should have a referendum on it?
 Apparently another reason for agonising is the concern that not everyone knows we are already a member of the EU...