First of all a country's constitution is sacrosanct and that constitution not only gives the people their power to vote, but also says that the country's territory is indivisible.
If the Catalan people want to change that, then the referendum should involve all of Spain's citizens, having as much rights at this point over Catalonia, as the people living there.
I've said it repeatedly here, so here it is again: I'm getting the feeling that people don't know what a democracy is.
A democracy isn't only about the freedom to vote. What happened in Turkey for example isn't democracy, even if a majority is in support of the current administration. The right to vote means nothing without liberal freedoms or without the indivisibility of the territory.
In other words, you can't have an "independence" referendum without involving all the citizens. That's not democracy, but a war declaration.