Re: What I don't understand
While I agree with what you have said in principle, I don't think it would be just in this case considering the referendum to enter1 the EU (or EC at the time) was done under simple majority as well. To me it would not be just to use a simple majority to enter, and then require a supra-majority to depart. In hindsight (being 20/20), it perhaps should have been a supra-majority to enter as well.
1 OK OK, to get technical, a referendum was held 2 years after the UK had joined the EC to determine whether the UK would stay in the EC. But the point stands that it was a simple majority vote.