Whatever the level of support (and we will really never know with any certainty, considering the referendum was backed by the very state that has now annexed Crimea), the point is the same. The sovereign territory of nation states are supposed to be nearly-inviolable against annexation. Special cases exist where the population of a region has been heavily persecuted (ie. Kosovo, South Sudan, East Timor), or where the situation between the governing power and the political entity had been as a client state (ie. Marshall Islands, Namibia). In a few other cases there has been mutually agreed upon terms for separation, as with the splitting of Czechoslovaki into the Czech Republic and Slovakia.
Merely having a large number of people of an ethnicity in a specific region is not in and of itself an argument for secession, and most certainly not an argument for a seceding polity to be annexed by another power. Now maybe the end run of a properly handled transition in Ukrainian politics may have been the departure of Crimea and other southern and eastern areas of the country, but the idea that there is any legitimacy to a referendum on allowing a polity to be annexed when the country that is to gain the region is the occupying power is absurd.