Let us begin with why the referendum should not be treated as advisory: the leaflet that Cameron sent to everybody.
Think about that a moment...
If we accept that, then that means that the PM can overrule Parliament through printing a simple pamphlet.
1/ "But the pamphlet made it absolutely clear", I hear you say, "and that was the basis upon which everybody voted."
That is true only for the people who do not understand the fundamental principles of representative democracy. Everyone who does knew it could only be advisory.
2/