Through article 50 of the Maastricht treaty:
www.lisbon-treaty.org/wcm/the-lisbon-treaty/treaty-on-European-union-and-comments/title-6-final-provisions/137-article-50.html
As they don't have to honour a leave vote, and the government could just leave anytime in the future, today's voting may as well be asking "do you like the EU, or not?"
And that would be a whole lot easier to answer!