It was not decided that if there would be <60%, there would be another referendum or the other side will win.
Staying in EU is a horrible idea as far as Britian is concerned. If they don't have to pay to corrupt government officials of poorer countries in the EU, it is already a win (for citizens of those countries and for Britain)... but I can see why the corrupt unelected EU officials don't want them (or anyone else for that matter) to leave.